No experience with the heaters but my advice is just plenty of layers in bed, a hat and thick socks if you're a really cold type and a quilt on top of your sleeping bags really does make a huge difference. To give you an idea, I'm in peterborough (so not exactly on a cold coast or high area) and i have a campervan, we slept out on driveway the 3rd week of April, had been a glorious day but it does still dip heavily at night, i had on PJs, socks, a cardigan, dressing gown, sleeping bag and a quilt, then I woke at 1am and was STILL cold
But you won't need to worry about that as it'll be 30 degrees like it was last year!!! Can live in hope...
Also, don't think we do the communal BBQs anymore on the camping trips, not since the year before last .