My shower is losing pressure when I turn the heating up, I have been advised the valve needs to be replaced, how much would I expect to pay?

If you replace it yourself - it probably just needs the valve module from inside, not the entire control - then it will only be the cost of the parts, probably $50 or less.

What's happening is the anti-scald device within your shower control is not working correctly, and it has failed in a "safe" mode which will continue to prevent you getting scalded. Unfortunately that also means you don't get any warm water in your shower, so as a convenience feature it's a mixed bag. Some designs can be de-scaled (delimed, demineralized, however you want to say it), some can be replaced as separate bits (not generally) and some require that you replace the valve module.

I work in a large homeless shelter and we have a lot of Delta shower controls that, due to abuse, tend to fail pretty often. They aren't as bad as some I've seen (the even more trouble-prone Leonard controls cost $85 each time) at about $30, and it only takes a pipe wrench and a Philips screwdriver to do the job. Ten minutes, tops. Your mileage, as they say, may vary.

Normal cause of loss of shower pressure maybe due to calcium deposits blocking the spray holes and unlikely failure with your heating as your question.

An easy fix with the shower off unscrew the shower head from chrome flexable hose or from the wall pipe -dont loose any rubber sealant washers, soak it in white vinager or a kettle/steam iron descale maybe scrub clean the face of the shower head tiny holes depending on how much calcium deposits, rinse off especially run water into the water inlet and see that water flows out through all the shower holes. Re assemble and put the shower on to see if the shower pressure improves.

If no improvement of shower spray there could be a few reasons impossible to speculate without more information type of hot water system, see what the basin hot water pressure is like in the bathroom if it's strong enough to spray it could be a failure of the shower mixer valve, a new mixer valve is $30 fitted maybe $60 add if shower is tiled $80

£3000

Seriously impossible to guess with out real world info as it varies greatly depending on what country u live in and numerous other unknowns