sorry mjh, I don't know what a polytunnel heater is, so I guess the answer is probably no.
discount, the pump is before the heater, but if I use a bypass to put some of the hot water back into the tank, then i'll be pumping hot water back out won't I ?
lastly, ok cleaning, it was a bit cramped, now the gas bottle is underneath the ladders, as i've moved my 125 litre tank across to the right. it's much better now and as long as I can get my hosereel and pole in and out easily i'm happy