Thanks, Odders!
We're using plastic boards now, and they're closely washed and sterilised, and, 'touch wood', seem to be contributing to improvements - with no further details needed!
Incidentally, I kept the old wooden board (for kindling next winter - or next week if you like) outside in the woodshed, and it's now growing green mould along every joint in the woodwork...
Hmmm...