Well it has been a difficult journey but looks like I am nearly there.

Managed to find an old PC with COM port but had no USB so couldn't get ProntoEdit software across as no 3.5" floppy on my current PC's. Ended up burning a CD to do it and after a few false starts got it talking to the Pronto and then loading IR codes. Checking the KAONNS1120 codes against the ones from the www and they were close so corrected in TiVo and tried again. No luck. Spent a day or so confirming that I was changing the CableBoxCodeNum correct. Then fed the TiVo IR back into the Pronto to see the codes I was getting and they were different. Found the frequency conversion used in the Pronto to Tivo Code conversion spreadsheet was wrong. So putting in a number of random numbers and working out the relationship I got a correction and put this in and lo and behold TiVo is now controlling Sky again.

I did have to tape the transmitter to the front of the box but no shielding. I am getting some issues with multiple digits and I think 8 or 9 might not be working very well, but I am sure I can get these with some tweaking.

So I will see what it does over the weekend and see if I can fix the rest of the numbers but looks like there will be life in the old TiVo yet.