Valliant turbomax VUW282/1 e - Hot water problems?

Where to start -

When we turn on the hot tap we don't get hot water unless we turn it on full. It runs for about 30 seconds then goes cold.

We have recently had a new pressure release valve fitted and a DHW heat exchanger (for a different problem) but it did work fine for a couple of months after this.

Any help greatly appreciated