At a guess, the gap would probably be about 250mm? Centred, anyway. But haven't bought anything yet. I could alwasy get thicker ply, or maybe double up the current thickness.
I was also thinking about placing two pieces of wood (it looks at bit like chipboard, but it is smooth, and more fuzzy, if you know what I mean.) on top of the blocks, to help with the weight distribution.
And JV, Dad said that theoretically, I didn't need any wood under it, because it wouldn't be spreading the weight
Thanks,
George