I had exactly the same problem a while back when cleaning out a sand filled container. I tried WD40, a rubber sucker on the disc, washing-up liquid but nothing budged. I took the U-bend off and put a bucket underneath. I got a bamboo cane and got it in contact with the underside of the disc and repeatedly hit the other end of the cane with a hammer and hey presto it popped up. Once released I gave the whole assembly a good clean with methylated spirit. I replaced the U-bend and everything's been working well ever since.
