still 6 degrees below the Minimum survival rating of the sleeping bag purchased!
Temp ratings for sleeping bags are at best an educated guess - it depends on how hot the person sleeps, humidity, how good their groundmat is, etc. Some people might be fine in a 2 Season bag, whilst others need a 4 Season, on the same site at the same time.
The survival temp rating is basically put there by the companies lawyers to say that is the temp that you wont actually (probably) die. Are you going to be miserable at that temp? Almost certainly. Also remember that sunrise in Inverness last week was about 6.40am, so the air had an hour and a half to warm up. We drove back to Inverness in late July, and there was a hell of a lot of fog at 5.30am, with the sun up. Think what it might be like at 3am in a deep valley. I wouldn't have wanted to be in a summer sleeping bag even then, and certainly not now.
There is a whole thread on OM about temp's for sleeping bags for Scottish winter use -
http://www.outdoorsmagic.com/forum/gear/sleeping-bag-for-winter-use-in-scotland/28148.html , with loads of good advice, but if I'd be buying a sleeping bag for this time of year in that part of the world, I'd want a comfort temp of minus 6 at the least, and possibly minus 12. As I said though, its all down to the individual circumstances, mat, etc, but a night in a cold sleeping bag is a miserable experience.
A sleeping bag will be on the list for a reason.... Make sure its suitable
I think thats great advice.