How can i increase the water pressure on my worcester 350 combi boiler?

Sorry I dont know YOUR boiler, HOWEVER I expect it will have a filling loop underneath - maybe boxed in.

Look for a flexible tube (loop) often silver in appearance. It will have a valve at each end (or maybe just one). Open them to allow water into the system while you WATCH the pressure rise. Raise to 1 - 1.5 bar & shut valve(s) again.

Steve
March 2008