Once you've solved this problem, it's worth thinking about whether the
air-brick is blocked. I had a nest behind one air brick a while ago,
treated with foam and though nothing of it. Then the boiler man came
for the annual service a few years later and took a look at
ventilation etc - It turned out this was the way in for the boiler air
(odd system but then it's hot air, not a normal setup) and it was
comprehensively sealed, buggering the balance of the boiler. It
didn't take me long to fix it (cut the brick out, cleared the nest and
mortared a new one in) but it could have been pretty dangerous.