New kitchen, oven in cabinet next to the mains water pipe/stockcock for which the installer has cut an access next to it in the cabinet should we need it.

Its occurred to me the heat existing the back of the oven will be about 4″ away from it.

Could the stopcock be affected, if so how, and would I be better off covering the access with something like radiator reflective backing to protect it against the heat?

Nothing as copper is pretty good at resisting being mildly warm what with it being used to carry the water in your central heating[ or attached to the back of a wood burner] and having a melting point of over 1000 degrees C

Could the stopcock be affected, if so how,

Worst that could happen is you get a pleasantly warm hand should you need to use the stop cock

Given that kitchn cupboards next to ovens don’t spontaneously combust, I think it’ll be ok.

In the specification of the over the rear exit temp is specified as 70 degrees +/-2%. I guess I was more worried about the heat affect on the washer /ptfe tape then potentially causing a leak (its leaked before requiring a new gland/tap).

I might tape the rad shield over anyway just as a precaution.

The PFTE is good to ~200 from what i can remember and the rubber washer should be good to about 120 degrees, even if its the crappiest, cheapest nasty washer you can get.

Plus they are both connected to an extremely conductive pipe that will channel a lot of the heat into the surroundings (ground, house, water supply etc).

Biggest issue you’ll have is warm cold water.

Had that at a rented place years ago, the cold water supply ran through the wall behind the open fire. If the fire was running you either had to use the first cupful that came out of the tap, or run the tap for ages to get cold enough water to be pleasant to drink. If the fire was really going for it (most of the winter) you’d never get cold water.
So we put bottles in the fridge.