While waiting for the weather to warm up this morning I phoned cleantech to order some bits and bobs. I asked the Scottish guy
on the other end if he had ever come up with anything like an immersion heater to warm up your tank overnight.
Long story short I had a convo with a window cleaner in Worcester who puts a fishpond heater in his tank to raise the temperature a few degrees and who said that he was already out working today when other w'c'rs were sat defrosting their vans. He said just go and buy one from a garden centre and float it in the aperture of your tank overnight.
He uses 2 x 300w ones in his 800L tank and says it raises the temperature a few degrees and that the temperature in the van raises enough to keep his system frost free.
My nearest garden centre only had 150w ones so
I bought two at £17 each and they are floating in the top of my 400L tank as I type - I'll tell you the results for me tomorrow.