Linda, your title say Moody 33 Mk2 but your post says M31 Mk2, which are completely different yachts, can you confirm which one you are interested in. If its the M31 Mk2 and fitted with bilge keels hence the query about bolts, the tank is GRP and unscrews. Firstly the tank lid comes off but it means breaking the seal and then the tank comes out. If you just want a peep, under the sink we have a cut out were the pipes from the tank exit and there is a wooden cover that can be removed to peer under the tank.
Pete