Shower Either Hot or Cold After Turning Down Temperature on Boiler

[richyb]

Hi

My shower runs cold until the hot water kicks in and then is very hot. No in between.

I have turned down the water temperature on the boiler so it is bearable but it needs fixing. I've been advised that it may be the mixer valve which needs replacing.

I've posted a picture of the valve - can anyone advise if this is simple to replace and if so which valve should I buy? Want to avoid the expensive callout costs of plumbers if this is a quick fix.

Thanks in advance.