No i didnt get an l5 i had a morco heater which apparntly should of been better than the L5. I dont know, unless i was doing something wrong. When it worked it was fab and very hot, even after 100m of hose laid out, yeh youll always gonna loose some heat, but never enough for it to freeze lol...not that quick anyway. And a quick blast of hot through it son sorts it.
I had thought about your idea matt, as i said i filled with hot and it stayed hot for ages. No where near as hot as out of boiler though, but a nice working temp all the same. Let us know how you get on matt.