sam, it's a complex old game, banking. 6.7 down to 2.9 sounds like a good offer, doesn't it? Lower monthly payments, more holidays, a better car etc.
But, and it's a big but, adding hundreds of pounds in fees to your new mortgage can leave you much worse off in the long run, as you'll be paying interest on that money for donkey's years, and I doubt that your cheap rate from Abbey will last for more than three of them.
The moral of the story is, don't sign on the dotted line until you've talked to someone who really knows what the score is.