Henry, I'm guessing your ply lining stops short of the roof then

My advice would be to get yourself a couple of rolls of loft insulation and fully ply over the roof, with insulation all round. The roof is the most important bit. Putting it behind the existing is easy, doing the rest and joining it up takes a bit of messing about but it's worth it.
The air temp at mid height was 13 a few minutes ago, I put a thermometer by the solution outlets of the machine and it reads 2, quite a difference because the machine is in between the heater and outlets. Since the cold weather one of the outlets has been dripping when the pump is turned on, so I guess it froze, I wasn't running the heater on full at that time.
I might put my little 8" desk fan in there tonight on low to blow the air about, see if the difference is so great in the morning...