[There is a theory afoot that the Warhammer Fantasy and 40K worlds are typically a few inches of topsoil laid on top of a couple of feet, or more, of skulls - either that or there are naturally occurring open-cast skull mines. Either way, this doesn't match the geology of the Iberian Peninsula!]
Managed to unearth some lightweight filler, so these are now out in the workshop (which is fortunately warm today) to dry.
The only problem I have left is the one board that the previous owner had started work on, which has been grassed, but sadly not in the colours I want, and a bit too evenly, as I'm looking for something more sun-bleached in general.
Anyone any good tips on removing PVA-attached grass? My immediate thought is get it very wet.