Been a number of threads on this (use search)
One thing to note, I'm almost certain that the small print says you are supposed to have a full working bike so if your getting just a frame I'd keep it on the quiet.
Also been alot of discussion over 'topping up' and the rules here seem to be flexed quite alot. I think you should be asking your company whether they allow it rather than the bike shop (who want the sale)
My company don't allow it, whereas my LBS would have been happy to oblige.
I also did not get anywhere the savings quoted, in fact after the 5% 13th payment I paid in the region of £800 for a £1000 bike. I was probably in a really unlucky salary band though. The new calculator on the C2W website states I will get a much better saving should I choose to do it again this year.