Problem with Valliant Turbomax - varying water temp?

Hello all. Not sure of exact boiler make, but it is about 10 years old. Recently moved into new place, had boiler serviced immediately - given clean bill of health. Recently the bath hot water comes out hot then falls to tepid. All other taps (bathroom basin, kitchen etc)give out nice hot water. Plumber just been and said it was limescale build up on heat exchanger that meant couldn't cope with large volume of water out of bath tap. Fitted new one and same problem. Now he says it must be the diverter value. I'm racking up the costs here.....can anyone help? Am I being taken for a mug??