Falcon Combi-boiler model 9911L81521 has poor flow on the hot water?

I recently had a workman turn off the water in the street while someone in the house had a hot water tap running. There was a big clunk sound as water went off at that tap. Cold water pressure from street is good but the hot water flow is very poor now. I have tried dispelling air out of the system but to no avail. Can anyone please advise me.