Watertank inspection lid

I have a Oceanis 350 from 1987, which has 2 watertanks under port and starboard bunks. The inspection lid on one of them has cracked and needs replacing. The opening is 145mm across and the actual lid is a red screw type lid, very much like the ones you get on flare containers and the like (unfortunately, not similar enough). I tried Ancasta, who couldn't supply a replacement. Does anyone know where to get one of these?