FWIW I'd suggest looking at what Rating system is used, I find 'Leeds Uni Ratings' are optimistic for me, I seem to fit the EN Standard better.  Reading the PDF on the Mammut website would suggest a 6-7 deg difference between 'Leeds' rating and EN rating with the EN rating being more conservative.

http://www.mammut.ch/images/Mammut_Sleep_well_pt1_E.pdf

See page 30.

Hello,

         Healthy sleep fine at home and the camping barns have big mats in them. The same as big blue thick gym landing mats. My real concern is when staying in barns which are heated by electric heating and then changing to no heating I want a bag which will keep me warm but I don't want to be overheating. I have never had a down bag before so I have no Idea how warm they really keep you.

Being too warm is less of a problem than being too cold.

Most good sleeping bags have an 'almost full-length zip', which stops about a foot above the end of the bag. If it gets too hot just use the bag as a duvet. Take a cotton bag liner to use as a sheet over your mat. It's a much more comfortable way of sleeping than forcing yourself into a restrictive mummy bag.