I must be the most hamfisted person ever but I have managed to break the bolt (well it no longer tightens) on the seat collar.

Can I fit a new bolt or shall I get a new collar, if so what size would I need for a 27.2 seatpost?

thanks

sam

Generally the bolts are stronger than the clamp so most likely you have stripped the threads from your clamp.
Most common size to go with a 27.2 post is 31.8 clamp but its not always the case.

It depends on your frame, you need to measure the outer diameter of the seat tube (bit the post goes into). There are standard sizes but one manufacturer may use different tubing to another so you cant really judge from the size of the seatpost alone.

Yep the collar is fine, its the bolt and the internal clamp. Can I get these separately or do I have to buy a whole new clamp?

what do you mean by intrenal clamp? is it aQ/R (quick releae) seat post clamp? there a quite a few different design, some non Q/R have the bolt screw into the the alloy clamp, others have a short rod that has a threaded hole the blt sctrews into - the latter should be replaceable.