huge O2 bill and 22GB usage.

Agree with above, some nasty shocks can occur. IMO you should get rid of the dongle, they suck for speed and data allowance anyway. Get a proper broadband in, they your limit will be huge and you wont need to worry about this sort of thing.
 
by any chance you did not start using netflix. my usage sky rocketed when starting using it. but i knew to keep a eye on it.