I've asked the mods to merge this thread into Paul's earlier one.
I've figured out why the mould has appeared. (I think). When I walk the dog in the woods, I pick up a couple of large fallen sticks and drop them near the shed. When I've got a few, I crop them into 8 inch lengths and store them on dexian shelving in the shed to dry out. This keeps me with a good supply of kindling.
Some of this wood may have been old and rotting.
Anyway, at Mrs. B's suggestion, I cleaned the saddle with witchhazel. Worked a treat.