I have just bought a house
Fitted is a Rheem MPI 325 outdoor mains fed water heater
When I turn any hot tap on in the house water flows at a good rate but as the hot water starts to come to the tap the pressure is gone and the water is just a trickle

Hi Grimbo

Check to see if the shut off going into the water heater is fully opened... sounds like it has been partially closed.

Back to you...

Mark