hope you all had a great Christmas.
I got a problem with my Profi-heater, has anyone had any experience with these

It's a 3kw heater, basically it doesnt seem to be heating the water up.

i've stripped it out, and everything seems o.k., all the electrics seem to be connected o.k.,
the thermostat clicks off at about 8 degrees (my pond is currently sitting at about 11).
i had it running with the thermostat at 24 for 72 hours with no increase in temp of my pond.
it's alot of expense to have it on, on the off chance that its might be working intermittantly!!
i cant tell you how old the heater is as i was given it. but it doesnt look that old and the previous owner said it only run for about three months before they realised they couldnt afford it.
Any ideas? or has anyone got another one just lying around i could use/buy/borrow, just to get me through the winter?