Hi, I've finally succeeded in doing this I think, and thought I'd share what I learnt in case it helps anyone else out in same situation who is similarly tech-inexperienced.
First the most important thing: if you have an old Sky box (eg: mine was an old Amstrad), do not let it cool down! My drama started because I was cleaning behind the STBs and rearranging, so unplugged it... 20 minutes later when I tried to switch the Skybox back on I couldn't get it out of standby. I looked online for help and only then discovered this is a common and fatal problem. I tried advice to use a hairdryer to heat the Skybox up to no avail... the next suggestion was to open the box and replace the capacitor(?) -- which I took as an omen to switch to Freeview.
This started out straightforwardly, thanks to the advice on this forum about which kind of Freeview STB to get. I went with a Sony VTX-D800U from Ebay. Then I discovered it was receiving no signal. Yes, it is daft but never having had Freeview I'd just assumed it was 'over the air' like wifi and you didn't need a big external aerial. Turns out you still do, even in central London. So there was a few days delay + expense to get someone to install it. Thank god, that happened this morning just in time for the Olympics, at which point the next problem phase began -- getting the Freeview box to talk to TiVo.
My next stupid mistake was assuming I could just change the STB type in the TiVo settings to "Sony". That led to an hour of frustration wondering why none of the IR codes worked. At one point I even convinced myself the IR blaster cables were broken because I couldn't see any light flashing from them (!), so switched to the TiVo front blasting -- still no joy. Only then did I spot that TiVo was still talking about satellite set top box in its screen instruction... could it be that Sony doesn't have consistent IR codes?
After looking through TiVo menus I couldn't find anywhere to tell TiVo I had a Freeview box, so reverted to Guided Setup. Then got hit with phone connection hassles. I could not get it to connect. It was dialling, I could hear a dialtone when I plugged in a phone to the socket, and I tried it using 3 different phone cables, to no avail. Then I saw the photo in this forum that showed a number in the dialling prefix bit. I'd stupidly left it blank assuming it already had the number in its system from having been dialling it previously. The problem persisted. I wondered if it was dialling too fast, so put in a 'pause' before the number. Still no joy. I wondered if it was just too busy, so tried swapping and using the other numbers. I literally spent 4 hours pressing 'retry' every 5 minutes. A few times I managed to get it to connect but then it'd drop out saying 'interrupted'.
The worst thing of all was -- once you're in Guided Setup, you can't escape unless you make a successful call!! Even pulling the plug out doesn't get you back to the normal screen. I was at the point of giving up when I wondered if perhaps there was some interference somewhere (which would be odd given it had made calls successfully last week). But just in case... I turned the power off on the broadband modem, pulled the normal landline out of the wall, switched off every computer and mobile. Retried and magically the call finally worked. I got through to the bit where I could choose the type of box I had, tried the DTT option and then aha, Freeview, and then when I picked Sony the IR worked!! From there, it was on to downloading the programme data, and now it is doing its 8 hour index thing but I am hopeful it is sorted. And at least in the meantime I have back my 'now playing' shows and can use TiVo to drive the Freeview box.
To conclude, I know my problems were caused by my own stupidity, thus won't be encountered by any of the tech-geniuses on this board (of which I assume are 99%). But just in case there's anyone else of similar un-techiness who has to go through the Sky to Freeview switching, hopefully this will spare you some of the frustration.
And thanks again to everyone who is behind the AltEPG project. I adore my TiVo and am hugely grateful you have been able to keep it going.