They seem to fit in mine OK, I've tried two so far but these are both forward facing seats for ages 1+, I've not had any need to try the rear facing ones for babies - although that could be an issue in the next year or so, so I should probably give them a try but I can't see any reason why they wouldn't fit.
You will find some seats won't fit, but you get this with all cars, it's not the bucket though that will stop it (that tends to help as it gives the base of the seat more support) it's the length of the seat belt receiever socket bit. Some seats are designed for very small ones (they tyre buried into seats) and some are deisgned for the loose ones which are on a length of seat belt.
If you go to mothercare they will try a selection of seats for you to find the best one.