Many thanks to you all for your kind wishes and for the suggestions for the baseboards.
As far as what went wrong, I can only think that the coat of masonry paint stopped the boards from 'breathing' and expelling any moisture they might absorb. We live in a Cob house and I know how important the use of lime products are on it to allow the moisture that gets in to be able to escape again. I surmise that the trapped moisture froze and forced the boards apart.
Having said that though, the SBR that holds the ballast in place will also form a moisture barrier. I really don't know what caused it. Maybe it was the Pixies that plague my garden. Every time I buy a new plant for the garden it's a battle to keep it alive/in the ground/or to not be eaten. Yes, the more I think about it, it has to be Pixies

.
As Mr. 40057 suggests, it might be better to consider a solid wood rather than a ply. We have a company in our village that makes preformed wooden houses so I guess they would know a thing or two about outdoor wooden structures.
I did build a Summerhouse for Mrs. Phillips (just before I asked if I could have a garden railway

), and that is still standing. The roof was made of a compressed wood board and that is only covered with roofing felt and the adhesive for it. Granted, it does have a pitch that my railway will never have, but it seems to work so it must be possible.
I'm starting to ramble a bit now so I'll sign off.
All the best,
Keith