I wouldn't as any salt in there will trap between and cause corrosion. 

On the subject of the type of chequer plate, some suppliers stipulate “marine grade”. What does marine grade denote in terms of spec and quality?

Secondly, there’s clearly a balance between thickness and weight. One of the motivations is to cut down on weight, so would 2.5mm be thick enough…….supporting stringers are 20cm apart.
Cheers

Next question - is the use of ploystyrene blocks under the floor for sound dampening, or insulation or both........or something else? Just lifted the old wooden flooring and there are fairly random bits of polystyrene beneath. I presume just put back and fill the gaps with new??

Yes was for flotation. I think back in the day it was an easier sell if the hull would float. I removed all of it on my Ramco as the salt would build up between the blocks and the hull. Stuff looked like soap but wasn't. I'd suggest if you are keen to leave it in there then wrap it in cling wrap to stop contact to the hull. Also beware when adding more as some polystyrene isn't waterproof. 

Thanks everyone for the replies. I think I'll leave the polystyrene out of the equation .

I wouldn’t all boats need flotation.