On Sun, 14 Mar 2010 11:52:25 -0700 (PDT), NogbadtheCool
wrote:

bit late for this year I suppose, but I have a thermostat and a length
of plastic wired with heating cable which I keep meaning to make into
a proper heated seedbed.

Don't believe its to late, my three will be in full use for the next
six weeks at least.

Previous posts are correct, but I also line mine with that reflective
thick foil to keep the heat in, also the sand MUST be kept damp for
the cables to work correctly, some can burn out if they get dry.

So Heavy damp sand, hence the weight problem.
And to keep the sand really damp, cover the surfuce with a sheet of
polythene, otherwise you will be having to constanly re wet.