Worcester 35cdi II problem?

Help ! I've got a Worcester 35 cdi II which seems to be ovepressurising when there is a demand. When cental heating is off and no demand for hot water, pressure is approx 1 bar. When c/h is on pressure rises to 3+ bar and water leaks from pressure relief valve at the rate of about a bucketful every couple of hours. I don't think this is normal. I have checked the expansion vessel and there is about 1 bar pressure. Anyone any ideas please? Thanks in advance