glowworm betacom 30 problem?

Hello folks,

boiler is 2 years old, had plate heat exchanger cleaned/changed/cleaned...dhw sensor replaced, hot water on...off...on...anyway, friend of a friend arrives, says system sludged!! powerflushes system, adds a magnetic filter to return pipe, hey presto hot water and central heating work!! kind of. heating comes on, ramps up to 70 degrees, goes off before rads are hot, he reckons boiler is oversized and should be range rated down but needs manufacturers instructions to do it, he's also said i might need new pump, air vent, expansion vessel as sludge build up was intense. i saw the amount that came out of system flush and it did seem alot. Am i being taken for a ride? he's already charged me 250 quid for powerflush and supply/fit magnaclean to return pipe.