Nigel did you make the panel yourself -15mm -22mm tube or is it a bought evacuated tube type, and is the cylinder a single coil or a dual coil,  these are dear !!!!! been looking for a while now for one of these to work with a wood burner.
I have just won 2 x ultra violet filters from work this week, and will hope in time ;D to be using 1 of these in a collected rainwater system,I have a bit of a query that might be in your line, as I have also collected a few 24dc shurflow pumps which I was going to incorporate in this system.
, but not sure about flow rate going through filters ? I know there is a max but cannot find info on minimum? any advise appreciated.
Thanks Phil