go onto freeview then press menu and installation should come up press this to see what happens and it should probably come up with automatic search press this and it should install all channels available
Thanks all - I've just realised that Freeview is actually received through the aerial, and I've checked and discovered that we only have a 17% signal through ours, which is probably why we can't get all the channels.
Our next door neighbours, when having their loft extension built, thought they were being helpful when they twiddled our aerial around - which is probably at least part of the reason. As we have digital TV we hadn't thought to do anything about it, but I think we probably need to do this so that we have some kind of TV back up.
Virgin Media have been absolute hopeless about this - they can't send us an engineer for almost a week to deal with the problem.