Finally sorted this out, my install is booked for Saturday.
Phoned up customer services and said to upgrade and the guy initially started quoting it would be MORE expensive for me! I'm currently on the old XL Plus scheme (50 meg bb/1 x V+ 1 x standard box, XL phone) which as stated was phased out years ago but they recently jacked the price up to £88 (I was initially paying £66).
I had to actually mention the premier collection and build your own bundle page specced 100mb premier for me at £66.15 (he was trying to say it would be nearer £100 p/m!)
Just shows you need to have facts to hand when phoning customer services otherwise they will walk all over you.
There is a £49 installation charge for the Tivo but I couldn't be bothered to try and argue with it. I'm saving £22 p/m and getting Tivo and 100 meg on top of what I've got now so I'm happy. Tying me into a 12 month contract but I can't see me going anywhere