At last a day that has been dry enough and warm enough to enable me to get the ply of the outer floor sealed. Having spent quite a lot of time exploring the wisdom of wooden boat builders on various websites and forums I'd decided that I would take the extra precaution of sealing it with a two-part epoxy sealant even though the ply claims to be good for exposed exterior use (i.e. conforms to national French standard CTBX). But, like all epoxys, the stuff I bought, called Resolcoat 1010, came with recommendations that it should only be used in temperatures between 18 and 25C and when humidity is relatively low. Easy enough to manage if you're working in a building but my 'workshop' doesn't have walls or a roof so I've been waiting for the day. And today was it!
So, in a perfectly acceptable sunny 20C, I've lashed a heavy first coat onto the ply and then filled the countersunk screwheads, also with an epoxy, before the sealant has fully cured - another tip from a boat builder that I found somewhere. It apparently means the filler will bond better than it would to the raw ply.
Apart from the fact that I mixed up twice as much sealant as I actually needed (And at 40€ a kilo I can't afford to do that very often!) it all appears to have gone as planned and so hopefully, given another nice day tomorrow, I can get the filler sanded off and a second and possibly even a third coat of sealant put on.