Worcester oil heatslave 20/25 about 4years old. ?

When i ask for hot water I get the pre-heated hot water but when this is exhausted it runs cold. The boiler still runs in an attempt to heat more water but at the same time the radiators get piping hot. Is it a problem with my diverter valve or the diverter valve actuator or both. How can I keep the cost of the repair to a minimum. The valve appears to be a honeywell and the only part number visable is 4004797-003 which I cant seem to find anywhere. Having read the instructions I turned the power off and moved a manual lever on the plastic actuator to the mid position but when I restored the power it stayed put and did not return. Any ideas please? as running a bath is not possible.