Update: I stumbled across this thread http://tinyurl.com/j86v2 (warning
this is a long read!)

Basically this guy's problem was very similar to mine (I also have a 6m
head pump set to max), in fact i had tried many of the steps he had
(conversion to combined feed & vent etc) also without success, before
going sealed which resolved the air issue, until we had some work done
in the house five months or so later.

After reading this thread I did recall that after the plumber converted
to a sealed system i did notice the cold pressure was quite high, near
1.5 bar, whereas I had been pressurising to around 1.25 bar cold. So a
couple of nights ago I increased the cold pressure to 1.5 bar and
hey-presto, problem solved immediately, no more air.

Whilst i'm relieved that the problem has gone away, I am perplexed as
to how you could have a leak that doesn't let water out, but plenty of
air in unless you pressurise to 1.5 bar? Bear in mind this issue wasn't
a small amount of air, but what seemed like loads, ie the system made
noises you associate with venting a refilled system every time I had
the hot water circuit on by itself.

Any ideas? I don't really feel like I've solved the problem, just found
a work-around.