I'm in Australia and have an 80 litre electric HWS . Suddenly the water is only warm with no cold needing to be added for a shower . If I turn the hotware on in the kitchen , the shower water becomes very hot and needs cold water to be able to shower so it appears that the HWS is working but not delivering... I'm in the country so getting a plumber is not easy... any clues would be appreciated

Roger

Assuming you have a clothes washer, try this.
Close one of the valves to the clothes washer, either hot or cold and see what happens.