The trouble starts pretty much straight away, as the underwater footage in the 2.35:1 broadcast looks grainy and, worse, plagued by clear evidence of MPEG blocking noise caused by video compression techniques.

For the most part, this blocky noise appears fairly random over the screen, but sometimes it also manifests itself as some really quite ugly 'striping' of what should be smooth colour transitions between the different blues of the water.

It's hard to say whether these compression artefacts are caused by Sky reducing the film's bitrate for broadcast purposes or whether they're present on the original HD remastering job undertaken by Fox. But their presence is undeniable, and really disappointing.

My subjective impressions that Titanic is having its video compressed quite heavily versus the situation with Zulu and The Italian Job is supported, furthermore, by some pretty simple empirical evidence. For even though at 213 minutes Titanic is substantially longer than either Zulu (143 mins) or The Italian Job (123 mins), it only uses up around the same amount of the Sky HD receiver's hard disk memory: 13%, versus 12% for The Italian Job and 14% for Zulu. Had it been shown using the same data bandwidth that appears to have been used for Zulu, we surely would have been looking at a file size near 17 or 18%.

To be fair to the Titanic HD broadcast, the MPEG noise problem is much more visible during the underwater sequences than during the film as a whole. But it can also be seen to a lesser extent in background walls and even, occasionally, people's faces.

It's possible that the same compression is responsible for the fact that the Titanic picture seldom looks as sharp and detailed as Zulu or The Italian Job.

I'm not suggesting here that the picture doesn't actually look high def - it does, at least at times. Or at any rate it looks sharper and more detailed than the DVD transfer of the film. By way of a suitably trivial example of what I'm talking about, you can make out the individual fibres in Bill Paxton's suitably sea captain-esque woolly jumper as he chats with the 101-year-old version of Rose.

Yes, yes, I know; here's one of the most visually dramatic weepies every committed to celluloid, and I'm banging on about a woolly jumper. But that's what an obsession with HD does for you, I'm afraid!

The colours of the transfer are quite natural and rich too apart from when the MPEG noise causes the occasional blemish, and there's practically no trace of any dirt or speckling from the original film master.

I should probably also add in the broadcast's defence that under water footage is notoriously tricky to show pristinely, and that some scenes, especially early on, are shot in a deliberately soft 'reminiscence' style.

But in the end, while I might try to make excuses till I'm as blue in the face as 'frosty' Jack near the film's end (sorry, bit sick that), there's no doubt that this Sky HD Titanic showing is certainly very far indeed from being the HD demo material that we'd all have loved it to be.

The bass channel, for instance, puts your subwoofer through its paces in quite dramatic fashion over the course of the film's latter hour, and these same sinking scenes also feature plenty of the surround sound gymnastics that have become trademarks of James Cameron films. But don't worry, slush fans; no matter how suitably titanic the din behind them, voices always sound clear and believable over the centre channel ensuring that you never miss a word of the over-sentimental script.

The score sounds irresistibly heart-tugging in its surround sound incarnation too. Listen carefully and you can't help but appreciate just how masterfully it has been put together to manipulate your emotions.

The soundtrack certainly isn't perfect, though, despite winning numerous awards. For instance, I personally would have preferred a little more audio effect subtlety in place of the sheer bombast of the final sinking scenes, and also I felt that during the film's quieter times slightly more effort could have gone into building a richer sense of being aboard a living, breathing ship full of people.

Finally, although it's minor, there are one or two moments where people's lips move slightly out of sync with the soundtrack.

But ironically it's precisely this love of the whole Titanic AV experience that makes me feel so underwhelmed by Sky's HD premiere of the film. I'd expected a slice of HD magic that could recreate the jaw-dropping spectacle of watching the film at the cinema, but instead I got a picture that for some of the time at least actually looks only marginally better than a standard definition DVD.