Well, for starters I'd look else where. Also, Just Nova's do it for about £3k. They specialise in C20XE and C20LET conversions for Nova's and Corsa's. You may be paying a bit more for an F28 though.
If you aquire knowledge and stuff then you may be able to do it yourself. I am in the middle of doing a V6 conversion on my Astra. I'll be able to do it for under £1500 all in as opposed to other places which charge close to £7k for a V6 conversion.