Chris, this stuff can be a real mare but you have a straightforward enough set up. I have Sky & NTL digital boxes, plus a pass-through NTL RF feed, these all feed my TiVo which in turns goes both directly to the telly and also through the home theatre system (which is a two piece - complicated, from Panasonic). I then take all signals from every device (including the DVD) and pump them round the house to 3 other tellies.
I have no idea how it is wired. I get it all working solely by trial and error. The plan which draw each time I have to disconnect it never makes sense next time I get it out.
Termie seems to have an inkling, the scart switch may be the key - I don't use one of those and I'm now wondering if it might help me.
btw once you get to multi devices it is certainly worth investing in a good programmable remote. I bought one for about £70 and it can do everything and anything which the originals can do (it has "learn mode"). You can link functions (for instance one button closes down all my kit), and write macros to emulate multiple key presses, even from different remotes (e.g. my "On" button turns on the telly, then the DVD,. then pauses for some seconds as it behaves differently depending whether there is a DVD loaded or not, then starts all the other devices, then puts them all to the same channel setting and sets the volumes to a preset level. The whole sequence takes about 90 seconds - but would be a real pain using individual remotes.
