1. go to B&Q, buy some "High Modulus" bathroom sealent - bout a £5 a tube - it must be high modulus so as to allow the seal to flex without breaking.
2. put a thin film between the mating faces, then bolt together, do not over tighten the bolts
3. then run the sealent around the channel on the back of the rim between the outer rim & inner rim, and force down into the join.
you can do step 3 alone if you just want to reseal them without having them apart
also, check the valve seating, put some sealent round back of it also as it leaks sometimes
