Unfortunately what's on preset 15 can be different depending where you live and how the autotune facility on your TV set things up. What TV station is normally on preset 15? (in many areas it's Film 4). Also do you know which transmitter you aerial points at?  

It's unusual with digital to loose just one preset channel so check which others are missing.  With digital TV one frequency (Transmitter channel) carries a whole set of TV channels. The set is called a Multiplex. If there is a problem recieving one frequency you normally loose all the channels in that multiplex as well.

For example in my area (Sandy Heath Transmitter) preset 15 is normally Film 4 which is part of Multiplex D.  Mux D also contains..

12 Yesterday, 15 Film4, 18 4Music, 20 Virgin 1, 21 TMF, 22 Ideal World, 25 Dave ja vu, 27 ITV2+1, 41 TOPUP Anytime4, 43 Gems TV, 45 Lottery Xtra, 85 Russia Today, 711 The Hits radio, 713 Kiss radio, 714 Heat Radio, 715 Magic radio, 716 Q radio, 718 Smooth radio, 722 Kerrang! radio

Are any of those also missing or unreliable?

Ok my guess is that Mux D is weak. It's possible that the reciever in the digibox isn't quite as sensitive as the one in the TV.

Care to give us your postcode so we can look up your transmitter?

Sorry yes I've been away visiting and only had rushed access on family PC.

The wolfbane site..
http://www.wolfbane.com/cgi-bin/tvd.exe?
suggests you aren't in a great location for digital TV. There isn't a transmitter it considers "local" but if you select the "distant" button it lists the Wrekin East transmitter which is a small 1-2KW transmitter. Is that where your aerial is pointed?  To recieve signals from there it predicts you would need an "Amplified extra hi-gain". eg A big aerial with a mast head amplifier might be required. Even then recieption might be a struggle depending on local conditions.

Here is a commentary on the Wrekin transmitter...
http://www.ukfree.tv/txdetail.php?a=SJ628082
Scroll down and you can see that that Mux D (containing channel 15) is probably the weakest Mux so perhaps no surprise that's the one you say is most unreliable.

It's a job for a good aerial man but he may get up there and say they can't improve matters.  If you already have a high quality aerial installation then I suspect Mux D might come and go until the transmitter power is increased after analogue switch off in 2011.

Thanks Cwatters.

The aerial is the proper digital one, fitted some time ago when we first got the digibox that now serves the DVD recorder. The TV is a 42" super-duper plasma screen and the digital reception on that is superb.

I do, however, think that you're right about having the aerial re-aligned because this is quite a high building and the aerial is on the chimney stack and gets a belting from high winds and checking it visually this morning it does look as if it's dropped from the horizontal.

Difficult to tell which transmitter it's pointing at because it's only a few degrees between Wrekin and the Sutton Coldfield one, but the engineer will sort that. When we were in town the normal TV came from Sutton Coldfield, despite Wrekin being closer, because the SC signal was that much stronger, better even than the smaller repeater at Knowle Sands which was only a mile and a half away. If you looked at the alignment of all the aerials in the immediate vicinity they all pointed in one of THREE directions.

Like you say, so many environmental factors are in play!

Thanks again.

My brothers digibox looses a few channels when his WiFi router is turned on.  He used to leave it on at all times, but now only turns it on when he needs it.

There are lots of possibilities... The TV band is pretty crowded and it was a struggle to fit in channel 5 everywhere when that was launched. Before digital roll out many TV engineers thought it would be impossible to do dual running of digital and analogue but actually it's working surprisingly well.

There can still be issues though. For example say you are pointing at Wrekin and it's broadcasting Mux D at 1kw on Channel 57. There might be another transmitter nearby broadcasting one of the old analogue channels on say Channel 56 but at 10KW or more.  The high gain aerial and amp you need for the weak signal from Wrekin might be swamped by the adjacent analogue transmission. The offending transmitter might be in the oposite direction to the one the aerial is pointing.

I'm not saying this is the cause (this is a made up example) but such problems do exist and most will be resolved when analogue switch off occurs. Personally I think a lot of people have already switched and they could have done it sooner.  After switch over all the digital channels change and that may bring new problems for some people who currently have perfect digital reception.

Perhaps try a Freesat box? If feeling rich take a look at the Humax Freesat HD PVR ("Foxsat" it's called).

http://www.which.co.uk/news/2008/10/humaxs-new-freesat-hd-pvr-rivals-sky-160215....

Last time I looked they were like hens teeth as there was a supply shortage.

Also one from Panasonic..
http://www.techwatch.co.uk/2009/02/26/panasonic-launch-duel-tuner-freesat-hd-pvr...

No Sky but no subscription either.

I plan to go down the Freesat HD route as soon as funds permit...