I reckon if the belt doesn't cause problems within the first few months it's not going to. It shouldn't be a complicated job to get right although I know the 1.4 ACT engine needs a bit of care.Would you be tempted to go for the £263 quote with the two year warranty as I think that is a good price.
Just make sure they actually change all the bits. The guy who bought our last car got the belt done cheap and they didn't change the water pump. When he asked why they said it didn't need changing. There also might be bolts that should be replaced with new ones, I don't know that for a fact but that can be the case.