We went at easter when we had the crusader...all I would say is take lots of warm clothes with you so you stay warm during the day...it's hard to warm up once you've got cold.
The first time we went was the two weeks just after that incredibly warm march a few years back...it was then freezing, raining and blowing a gale and we just weren't prepared...found the evenings were the coldest when we'd got cold during the day and were sat around in the camper (which was draughty)...toasty warm in bed though as we had the heated mattress toppers which were brilliant. The second time we went in March, we were much better prepared and had a much nicer time.
Just been camping in Scotland on the first week in December in our Raclet Trailer Tent. Like a few people have suggested, try and get an EHU lead and a couple of small fan heaters plus warm clothing and a couple of hot water bottles. We borrowed our ehu and fan heaters. We had a 13.5 tog duvet and our own body heat in bed ;-)
Oh, and i have brought two cheap bottle jacks, one for each side,, have got tyres up off the ground now , all for 18 quid too, poor old rango is proper hunkered down for winter now,,, roll on easter,, already got the week off booked
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.