No hot water with Worcester 28Si?

We have a Worcester 28Si and are now getting no hot water at all.
It started a while ago. The central heating would work for a while then the reset button would flash but we could get hot water from the taps when reset. A few days later the CH would work for a couple of mins and stop. We could get half a bath tub of HW before the reset flashed. A Few days later all we could get was a wash basin of hot water before it broke. Now we are lucky if we get a mug full.
I've checked thee Water flow turbine for debris, the water pump, the airflow pressure switch, nothing. The fan is turning, and i've flushed out the heat exchanger. But still not working.

Can anyone help as the missus is starting to moan.