Vokera Combi overheat?

I have avokera combi and it over heats, so far we have replaced the Expansion Vessel and the PRV, signeficant improvement but still it overheat. After a couple of days we checked the EV pressure again it was16 PSI but the boiler still overheat. Some suggest that I should clean the flexi pipe, any idea what is the reasonable cost for doing this?