Think back, if you will, to the trailer that began to show up on BBC ONE in early March 2005. You know the one.

D'you wanna come with me? 'Cos if you do, then I should warn you...

For anyone who had just about given up hope that Doctor Who would ever be seen on our screens again, that trailer was one of the most magical pieces of television in a very long time. A succession of brief, juicy snippets of series one action followed Eccleston's introduction, just enough to whet the appetite without giving too much away. And of the footage, one particular moment - save, of course, the Dalek shot at the end - attracted the most amount of intrigue and "Cor, that looks BRILLIANT!"-type excitement.

An alien space ship crashing into Big Ben.

An alien space ship. Crashing into Big Ben.

This never stops being absolutely fucking wonderful.

How can you not look desperately forward to a story that contains something like that? Of course, a lot was riding on Aliens of London beforehand for reasons that extend past a quick shot in a trailer - following an alien-packed trip to the future and a classy "celebrity historical", it represented the Doctor and Rose's first return to a contemporary setting, and the first opportunity for the new series to show that it could "do" a striking and relevant piece of action. Sadly, by placing the episodes once again in the hands of Keith "Rose" Boak, the new series' first "alien invasion" story failed to attract anything like the atmosphere and tone that The Christmas Invasion would later manage.

Story and script-wise, it gets off to a strong start, though. The frantic scene of Rose's return to the Powell estate is neatly played - although, while at the time the idea of exploring the effect on the wider lives of a whisked-away companion was a breath of fresh air, it looks in retrospect like an ominous portent of things to come (not that Jackie and Mickey didn't almost redeem themselves in series two's closing story - but in series one... kerr-ist...)

A fast-paced opening few minutes lead in to the absolutely wonderful scene in which the aforementioned trailer moment happens - and it really is terrific stuff. The crash itself is one of the finest moments in model maestro Mike Tucker's illustrious career, but the entire scene builds beautifully. The idea of alien objects over contemporary London has been re-used as new Who has gone on - but from the rocky majesty of the Sycorax ship to millions and millions of freakin' Daleks, it hasn't got tired yet. This, though, was the first time we'd seen such a thing, and without any knowledge of its dubious origins it was - and remains - an absolute delight.

Although it is preceded by a touch of weirdness - the rooftop scene, glossing over the ridiculous internet-fan-led anger at Rose's use of the phrase "You're so gay" (written by, of course, that notorious homophobe Russell Tee), looks quite peculiar nowadays when held up against the episodes that have followed. The matte behind the Doctor and Rose is so obvious, stark and bright in colour, that it really snaps you completely out of the scene when you notice it. A side-effect, perhaps, of the infamous "I thought it was just another kids' show" direction of that first block?

The exploration of people's reactions and attitudes towards a potential First Contact, however, sadly doesn't extend much past the first half of the episode, once the Slitheen plot begins to properly expand. It's a shame, as the odd moment in this episode - and indeed in The Christmas Invasion - show that there's some nice material to be had, and that RTD is perhaps capable of it. Unfortunately, once we get behind the doors of 10 Downing Street and the hospital, we're pretty much there for keeps.

And it is quite unfortunate when we're lumbered with the Slitheen for the bulk of it. They're not entirely without redemption, and they're mildly chilling when we witness their first transformations, but they do suffer from being somewhat on the silly side. The actors - and perhaps the direction of said actors - don't help this. I know that there's meant to be a lightness of touch to the way these baddies are presented - they're certainly the new series' most obviously kid-friendly monsters, and I mean that in terms of both the show itself and the marketing - but it's difficult to take them hugely seriously. And I don't just mean because of the fart jokes, either - although, while I'm not among those who were so strictly opposed to their presence altogether (they do result, of course, in one utterly great Eccleston line), they are completely overplayed. Compare Annette Badland's performance here, for example, with her one - playing the exact same character - in Boom Town. There's an absolute world of difference in the level of conviction.

The Slitheen clan aren't the only actors who let the whole thing down, though. The news anchor, in particular, is pretty embarrassing. Given the presence of Andrew Marr and Matt Baker playing themselves in the very same episode, would it have been too much of a stretch to just get a real one? It's probably not even the fault of the poor lad himself; but actors never sound like genuine reporters (unless they're Chris Morris), and this is a particularly unconvincing example.

"Yes, we know who you are."

Still, though, I'm surprised upon a recent viewing that my originally-held view of the episode - that it starts great and falters badly in the second half - is challenged slightly by the presence of some half-decent material in this latter part. The introduction of Harriet Jones of course takes on an entirely different level of meaning considering later events, but even without that context, Penelope Wilton absolutely shines here, making for arguably the new series' first genuinely likeable, engaging and convincing original character. Some of the scenes in the hospital, meanwhile, are genuinely tense - and again take on a new significance when the context, of their being so early in the shooting schedule and including Eccleston's very first recorded scene, is considered - despite the (again) inherent silliness of the pig idea.

It's the ending, though, that really sticks in the craw - and, indeed, that I think is perhaps largely responsible for the negative reactions to this episode's second half, and the storyline as a whole. The new series' first chance to do a proper cliffhanger is sorely, sorely wasted, with an absolutely tired and sorry drawn-out affair. It's not like the typical "monster descending on protagonist" idea couldn't be done convincingly by the new series - witness the zombies in The Empty Child to see how it can not only work, but be beautifully resolved - but simply that here, the tension is badly misjudged. You're praying for the scene to end long before it actually does, and when you're trying to create tension, then outstaying your welcome is a firm no-no.

Still, though, in hindsight, there's plenty to like in Aliens of London. The problem, really, is that expectations for it were too high - there were a lot of factors going for it that should (in the minds of rabidly anticipatory fans) have led to it being a great, epic invasion story rather than a bit of a knockabout, silly alien romp in the corridors of power. Conversely, though, the lowered expectations that now precede any viewing of it perhaps enhance the overall experience, as a general sense of "Oh yeah, I'd forgotten about that bit, it's not half bad, is it?" begins to unfold. But really, you can't help wondering if a cut-and-paste job of all the good bits of the story might have made for a much more compelling single-part story than the drawn-out and often laboured hour-and-a-half we ended up with.