Stating the obvious here (although David C has already done it) but you shouldn't just guess at the AF mix by partially draining the system and adding more AF.
If you don't know what the true ratio was to begin with, how do you know what the ratio is now?
Is the AF you've topped up with even the same type as what was in there?
I can totally understand the laziness at this time of year (with no garage to work in I've no motivation to get my hands dirty in such cold weather), but it's at this time of year that you should be taking this VERY seriously. I've heard of quite a few cases of people with frozen coolant in the last few weeks. Some have had lucky escapes, some haven't (broken water pump etc).
As said, you may have an air lock, but my money is on the coolant now having too rich an AF mixture and it's no longer acting as an effective coolant, hence the slight increase in running temp.