>>I also noticed last year during a particularly cold evening that I could feel some air coming in through a wall socket.

This is one place where electrical heating can really save you some money. The plug on the heater cord stops the cold air from coming in

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8