I have now fitted a stainless stanchion base. Reasonably straightforward: The penny washers and nuts for the old stanchion had been glassed in, and as the two outboard holes weren't in the same place so had to drill new holes, which slightly overlapped the old, not ideal really. Anyway squirted lots of silicone gell and rebolted reusing the penny wshers, but with the nuts backed up by further locknuts. I had to buy some new 'machine Screw Slotted Countersunk M4 x 25mm in A4 Stainless' bolts (2.46 GBP for 15 from Westfield fasteners) as the old bolts had been sawn off before glassing.
Hoping will stay waterproof! (however it is in the loo so not life and death if it leaks a bit). JimH