Thanks for the comments
Warming did not help
It is NOT all water (I too have been fooled that way in a dinghy gas tank some years ago)

Fuel source is local, used by everyone in the Clyde marina, so is not suspect
I am assuming we have a mild algae dose.

When we get afloat in a couple of weeks we will see what happened to the filters.
Fortunately I changed over to a twin unit which opens at the top for changing, so the changes are much easier than with the spin-on cartridges.