I have a grant combi boiler with a combimate and Mag One fitted it the system. Recently the Mag One started leaking. Heating engineer replaced whatever is inside and cured the leak. Since then, the shower is feeble and there isn't enough hot water to overcome the cold water pressure in the basin mixer tap. The boiler still fires up and gets hot, heating the rads but regularly overheats and cuts out. Could it be that he did something wrong with the Mag One? Any advice appreciated, thanks.