Pressure fault on Worcester 28Si II?

Hi. I'm repairing my boiler after it froze. I have the central heating working now (pump and flow sensor damaged), but when I turn on the mains water to the boiler it pressurises the system until it reaches over 3 bar then fires the blow-off valve. The loop pressuriser works normally.
If the water isn't switched on the hot water taps work (!) but uses up the heating pressure. This boiler is the one without the diverter valve, so I suspect there is a breakdown within the heat exchanger though there is no leak. Is this likely or have I missed something?
Thanks!