I agree 100% with that tamjam, having grown in the loft for 3 years through winter and summer myself in the past. The high temps they can handle but the cold REALLY stunts them. I remember you saying that your mrs was not keen on you using a heater up there before but look at those little oil filled rads that I mentioned from Argos because they are very safe and cheap to run and will mean you can grow up there through the coldest winter temps as I did. I used to have to put a fleece on or something to go up there but as soon as I opened up the tent it was warm and the plants looked very happy
Mine is a small 500watt heater and its still going strong. Theyre fitted with a thermo too so once you found the right setting you can hold hold the temps pretty steady up there in an enclosed tent or cab.
Just something to think about if you do decide to keep growing up there later.