I am sure I saw an add some years back for a small dehumidifier that
you can put under a cover of a boat in the winter - stop mould, damp
etc.
Anybody know of one - European preferable for 220V 50Hz, but would also
be OK if a 12V version.

I have seen 'heaters' to sue under covers, which claim to keep boat
above dew point .... anybody know if they work well, would assume a
unit that keeps air dry would be better choice, than one that just
keeps it warm.