Usually it's the seal on the 3 way valve shaft leaking. You can get a service kit for it for £10 or so, part no. 3003202082. If the actuator motor is corroded it might need replacing too, 3003200039. Without the actuator you will only get heating or hot water, can't remember which, best get it working properly before you fire it up again.
Andy
February 2017
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