Finally I see it all!
The initial problem was I lined my propagation table with heavy duty black plastic but each year it seems the plastic would puncture/rupture even though I took great care in installing and removing seed trays.
So I've built an inner lining from rigid white plastic, it was taking a lot of humping to remove about a 'ton of gritsand, the thermostat and the heater cable. Then replace the plastic then re-install the gritsand and 'stuff'. Hopefully this will last longer than just one year.
Cheers, Tony. :blink: