Did you go in via the partly flooded adit that's below the path up to Rhosydd? That's the way in to find all the goodies. Wade through the water until it dries out, continue straight until the partial collapse. Scramble through into the chamber, go straight across (past the 'shard' and remains of a tripod crane) to the tunnel opposite and after a few yards, turn right and follow the tramway track into the chamber at the end - one rubbish waggon and alongside, a slab waggon. Simples!
(If you're careful and feel for the tram rail head in the entrance to the flooded adit with your wellie, you can go crabwise along the railhead until the tunnel dries and keep your feet dry - walking on the railhead allowed you to keep within 'critical wellie depth' when I was there with Skimble and others last year...)