Andy, First thing is not to believe the ratings systems. They are designed to confuse you and a -2 bag will probably be just about warm enough for a comfortable night when it's forecast to be about +7 or 8 at night. Look at the label properly and you should see something like "comfort" and "extreme". The extreme rating is the temp you probably won't die in if you wear all your clothes and shiver all night. Comfort is the temp a fit young man (not joking) should have a comfortable night wearing thermals and fully fed and watered. There's an EN standard that should be quoted to cover all the above.
Also add in that ground temps can several degrees colder than air temps - which is what they forecast and is more noticeable when its cold - and its not surprising most people complain their minus 5 bag is rubbish when it gets below freezing.
So, if you really want to sleep at below zero temps wearing just a tee shirt you need a bag rated at -10 or lower. Down bags IMHO are loads better than synthetic not just coz they pack smaller but coz they drape better over your body are less clammy and so have a wider range of comfort, eg down might be comfortable from -5 to +15, whereas synthetic will be something like -5 to +10.
And before anyone says synthetic bags are warm when wet, I am sure they are a bit but a night in a wet synthetic bag is going to be a very miserable cold night. Time for a B&B I would have thought.
I have Nanok (-5) and Helsport (0c) down bags both of which cost me around £100 (the helsport reduced from something stupid like £250) and they are both brill,although I would say the helsport is warmer and nicer as it has better quality down in it and a better pertex outer. In warm weather I use a silk bag and the open up the sleeping bag as a quilt so I can just pull it up when needed.
I have also had several synthetic bags, each of which started great but after a few uses lost their loft and became cooler and not so nice places to be in. I am a convert to down now.
Good luck with the search.
Sent from my iPhone with a smile