I have an unvented hot water system.

Following a recently fitted shower I noticed our hot water all over the
house was coming out tepid--->hot.The worst taps are those nearest to
the shower. I cannot use the shower yet because the water won't come
out hot enough!

British Gas came out today and diagnosed back feeding through the new
shower.The elbow to the hot supply of the shower goes immediately cold
when you turn off the shower and then run the sink tap. If I run the
shower I can get hot water from the sink tap! Is this a faulty shower
with a dodgy non return valve or can it be fixed by reducing the cold
water pressure??

The engineers blocked off most of the hydraulic oil section of the
cooler and ran the test again. This returned fuel consumption to the
original level, but no significant improvement was seen.
It was concluded that the modification tested could result in a small
cost saving with respect to a Needle Valve reduction in size of the oil cooler. But
with fuel consumption being more important than any modest saving in
cooling capacity, the idea of paying more for a pump that resulted in
the oil being kept at a lower operating temperature