It seems people are crying out for hot water
we have the L5 i have made a post about my thoughts / idea's on that, the insulated tank would keep it warm for a while, look at your hot water tank in the airing cupboard
now we have all thought of using the 240 V heating element from the hot water cylinder tank, they are about 20 quid each, but you would need a stand for 1, not a major job, i am sure a metal fabricator would knock them up for you
the house opposite me keep tropical fish in a rather large tank ( it must hold 200 - 300 L of water ) they run a Aquarium heater ( search ebay , about 10 -15 quid delivered for a 200 W heater )
300 L will need 600 W of heaters to get to 15ºC warmer than the temp the water is
they are cheap to run and leave on all night
of course the main thing will be to insulated the tank well, this will help the water temps rise AND keep the water warm