Another lesson for Jimmy.
I thought the cover plate might just pop off but it's actually sealed with adhesive, bugger.
Anyway it wasn't really low on acid, the makers of them seem quite justified to seal them now. I made up bungs to reseal the filler holes out of curtain rod and rubber from an inner tube but I think I shall buy another and use this one for my low voltage house supply.
I seem to learn a lot of lessons from mistakes.