Glad someone is having more luck than me. Thanks for the IRSlice hint, almost got it to work. What I have done now is reprogram my Pronto using the IR codes sent out by TiVo to Sky and using that I get good results. This confirms to me that we are nearly there. What I did also find was that contrary to one of the links above you get much better results if the blaster is further away from the Sky box. I now have it about 100mm in front approximately in line with the buttons. This seems to work almost 100% for single digits and 50% for multiples. Still need to get multiples working better.
In case it helps I also attach the modified TCL file which for me gives near perfect imitation of the KAON codes as measured by the Pronto. KAONNS1120 new.zip
TiVo series 1 UK Thomson, Cachecard, 500GB, Sky Digital
TiVo series 1 USA Philips, Turbonet, 230GB, Freeview Dish TV