I've come to the conclusion that my plants are growing too slow because my grow box in the loft is too cold, being around 15 deg. C at night and 20 through the day.

I'm about to install a fan heater to help, but what temperature is it best set at? I was thinking of somewhere around 25 deg. C, but is this too high for a night time temperature (I've read that the plants like a temperature drop at night)?

A better solution I suppose is to keep it around 15-20 at night and 25-30 through the day. That's ok while I'm around to switch the thermostat, but I need something in place before I go away for a few days.

Any thoughts?

24oC day and night works just fine. If you want a temperature drop then I'd go for an 8 or 10oC drop but no more than that personally.

regards

Cheerss for that - mid-20s it is then.