Why does the water run cold?

I have a Camray Model 90-130 Combi, oil-fired central heating boiler. The central heating works fine, but when using the shower the hot water runs cold very quickly, even with the Priority Lamp lit on the boiler. To date,I have been told that the expansion tank and relief valve were the problem, and both have been changed by a plumber, but the problem still exists. Does anyone know what to look at or replace next?