Why is my Mira too hot?

The other day my hot water cylinder was taken out and a Vailient Combi boiler fitted , first my shower (Mira Excel) lost all pressure and the plumber took it apart to find lots of debree in the filters , after cleaning the filters out the shower pressure got better (acceptable- just to get the pain in the arses out my house!!)...., and since then the temperature has been the same - give or take a few degrees - its been really hot whether it be up at number 8 or down past the Blue setting number 1- (as you can imagine that first thing in the morning you think your skin will fall off with the heat!!) -- My landlord says its fine and should be that way-- i need some advice on what the problem could be , someone mentioned the cartridges inside may need replaced now £70 roughly ,what do you think?