If it's like a dust coating then you can suck it out with a pond vac

Thanks for the info, particularly the advice above. To be honest, I
didn't know that such a thing as a pond vac existed, but it sound like
just the thing I need.

Does anyone know if these are available from tool hire shops? The
couple I've looked up near me (in Berkshire) don't seem to list them.

Cheers

Will