My boiler clanks!?

I have a Potterton Puma 80e and some time ago I accidently closed a (air?) valve to my boiler which caused the boiler some - shall we say - consternation until I re-opened the valve.

Since then the pipes often clang and groan quite violently when the boiler is first switched on. It also seems to be allowing air in to the system and I find I need to bleed my rads frequently.

Is there anything I should be looking for? Can I find a pipe or system diagram anywhere which might help me when I explain to a central heating engineer what I've done. I don't intend to fix it myself - I just want to know what someone should be looking for.

Craig
October 2006