I have a problem with my rear seatbelt, its the middle one that's attached to the folding back rest.

i had my seats down and when i tried to lift the double section back up it wouldn't go, the middle seatbelt has locked itself with the full belt inside.

is there anyway to release the seatbelt so i can put my seat back up?

Ah! This has come up quite a few times. The middle seat belt is locked and unuseable for safety reasons until the back rest catch is locked into place so if you can simulate the back rest being up and locked by placing something of the right size in the catch then the middle seat belt will be released. Look for the backrest lock coloured indicator. Do a search and see if you can find the threads as they have helpful pictures in them too. If not then post up an alarm for more help by all means