isar he24 boiler?

i recentley had a service on my boiler which was fine, but i have a problem where as when the heating is on i cant get any hot water , i was told by the engineer that the valve was sticking and i would need to replace it at a cost of £106.00, would this price be right and could i replace it myself? also what is the correct name of the valve he is talking about, any help would be much appreciated. many thanks Mark