FAO Bobby Rob re sime friendly combi boiler?

you seem to know what you are talking about re sime boilers, can you help me with this one?
i have a sime friendly combi boiler installed in loft about 8 years old
i was getting intermitent hot water just getting to hand hot, but when the heating was on it was getting boiling hot.
called a plumber he said it was heating exchanger and would cost £260 to fix, he suggested that it wasn't worth spending the money on it because of it's age and suggested a new boiler. he has adjusted the heating excahnger to the maximum (?) and i now get hand hot water all the time, boiling hot when heating on, but i also noticed the other day that when i ran the water the radiators got hot even though the heating wasn't on.
in your opinion do you think it could be the heat exchanger or do you think i should get a second opinion,
PS diverter valve (?) was replaced decmber 05

your opinion would be appreciated