Any question answered...

At the moment I have a rack which is fastened to the rear brake bridge, I don't have canti mounts or rack mounts.

I've since seen that you can get racks which fasten to the seat tube or seat post - anyone got any experience of these?

Ok, if by that you mean cantilevered types where the ONLY attachment point is at the post or seat tube; I wouldn't particularly recommend these because of load limits (how much do you want to carry?) and the possible stress on your post or frame, especially if there's any carbon fibre in the mix. Look into Tubus, local and hopefully cheaper than the rest of us could get it for. Get some of this hardware and you'll be able to attach anything to anything.