Problems with the direct hot water from my Baxi 133 HE Plus ?

In order to get hot water (hot enough for a bath) from my Baxi 133 HSE PLus I have to run the water from the hot tap at a very low flow rate. As soon as I increase the flow rate the temperature drops considerably and if I try to run the hot tap at full flow the hot water temperature is just a few degrees above the cold water temp. Before this problem recently occured I was able to run the hot tap at full flow and the water slightly too hot to touch. If anybody can give me some pointers as to what the problem is likely to be I would be very grateful.