WOW!!
Just rang Virgin up to ask about the extra charge, told them that I was told when I ordered my TiVO that the charge was per account not box. (I ordered my extra TiVO back in August had to wait for an install in november). So he says one minute I'll check that now comes back to me saying we can can reduce your bill by £10 a month for the same services you have now.
I'm happy and pretty amazed by it all my bill will now be £69.50 a month. Until April 1st when it goes up to £71.50 with the price rise that's £2 on XL TV. That's what I was told. But that's still less than I've been paying.
EDIT: I have two TiVOs