Yet another combi boiler problem ;o)

Last week my radiators were getting warm when I ran the hot water so
called a local central heating company. A new divertor valve has been
fitted but then, the next day the system wouldn't light and heat hot
water, cut out valve thingymabob popped up and there was a terrible
banging noise.

Yesterday the engineer came back and told me it was air in the system
and he fiddled with it, bled it, topped up the water etc and ran the
hot tap for a while. Everything seemed fine, ran a shower ok this
morning but this evening when I ran the hot tap, same thing, banging,
water cuts out, valve pops up. That's when I noticed the pressure
gauge had plummeted from 2 to half way between 1 and 0.

I'll be phoning the engineer in teh morning but anyone got any ideas
what can be causing this air to get in the system? I'm happy to top
the system and it usually needs it a couple of times a year along with
the radiotors bled but I don't even want to think about fiddling with
it at the moment!

Any suggestions appreciated

Thanks


Lindsey