i'm not aware if you can ask for a particular box, but there are different versions of the same pace box. when the guy comes round to install it, give him a coffee/tea and have a chat with him about the boxes/future service/problems likely to occur - suggest u want the latest version of box - they usually have a couple in the van - also get him to set attenuation properly and test the input signal.
the older versions crashed a lot and often gave poor picure and sound quality - a revised version (lets say 1a) was then released and given to customers who had problems with previous boxes. then when telewest went "pseudo" digital just b4 the world cup (great timing - to sort out problems) the newer boxes showed some weaknesses too. a newer version was then given to customers (1b). when the telewest technician was last round (july) he said that there was a completely new silver version (with digital out) coming out but he said that it may be a while before telewest do a deal with pace because of the finacial situation, also tw will not broadcast dd/dts in the near future (6-12 mts).
tw also had 2 different companys producing the software for the digital service, 1 a local company i'm familiar with but not sure of the second - could find out if u really want to know. this then led to incompatibility problems and crashes of the customers box -software was not realising memory properly "a la" M.S. not that there is much anyway. recently 1 of the companies was/are about to be binned.
the difference between versions i'm aware off:
'1a' had more memory and a slightly different circuit layout
'1b' had a more 'direct' circuit board power and video outputs at opposite ends, more memory again and tweaking of the modem circuitery.
i've had some picture/sound/box problems over the last 13 mts since i've had tw cable.
the staff tend to be actually freindly/polite over the phone and seem to want to help but unfortunatley there has been problems on the software/hardware side and they are not very knowledgeable - also it seems the right hand does not know what the left is doing (i.e. sales and technical).
we were offered free installation and 1 month free to entice us to sign up - but we were actually overchraged above standard rates. took many phone calls and 6 weeks to sort out - very hard to get money back from them - they prefer to deduct fromyour next bill/s.
the only people who actually know much about these boxes/hardware/network are the senior "engineers" who come out to fix problems, the two that i've had round were very knowledgable and experienced, working previously with sky and in america sothey knew their stuff.
problems i've had at various time (resulting in visits 3 times over 13 mts).
box crashing
no sound
blocky picture/picture break up
box crashing/hang up - unable to soft reset
continous interactive problems - givin up on that
poor sound quality
poor picture quality
standard fixes from telewest helpline:
switch off box for 30s then back on (to reset/lear memory) - usually works.
switch off box, remove card - switch on they then remotely reset switch back off replace card the on again - done twice and worked.
check/replace remote batteries - juice hungry, worked
check scart leads - doh!
replace scart leads, even if they previuosly worked and were untouched -

sh*t methinks. but a good way to blag decent quality scart leads from tw engineers - got 3 - just incase
i've had technicians/engineers round to fix these problems - which resulted in replacement of box for newer version and increased signal strength/attenuation.
also if someone is using a chiped box or a modified analouge box on the same main "street box" as you then you will get poor sound/picture. the modified boxes create signal noise which affects your signal/reception. if u call up about poor picture/sound they send out a technician to test the street signal and see if someone is getting free channels, thats how they can catch you.
but there are ways round this
