Immersion pump not pumping hot water for long

Our ensuite shower is fed from a waterpump by the immersion. No matter if the tank (regular size) is fully heated the water goes lukewarm after 4-5 minutes max in the ensuite, while the main bathroom shower which isnt fed from the pump, could run at a nice hot temperature for a good 15 minutes with a half heated tank which is what Id expect.

It used to be ok and seems to only have been a problem over the last 3-4 months. Any ideas?

Seems like a problem with the thermostatic mixer. run the hot tap at the wash hand basin in en-suite next time to see if hot water is present.