Replacing a Bar shower valve

[rubyt1976]

We are in a new build (2 years) and the bar shower value on-off bit is jammed. I want to replace it myself as can't afford a plumber. The old one is is Aqualisa and has on/off on right and temperature on the left.
I have purchased a replacement bar shower valve with the on/off on left and temperature on the right as I understand that it makes no difference due to 'flow diversion'.

I have an adjustable spanner large enough to undo the screws to remove the old bar valve. I understand that I need to turn off the mains water but think that as we have a conventional boiler, I also need to turn off the water from the boiler. Please can you advise? TIA

 

[ShaunCorbs]

Do you have an unvented cylinder?

 

[king of pipes]

A few pictures would help us give you more accurate advice the normal set up is hot supply will be on the left cold on the right, if you turn off the mains cold water and run the kitchen taps and bathroom taps you should remove the water pressure and be able to remove the old shower valve and replace the mixer, but be aware things can go wrong even for us professionals so proceed with caution and allow yourself plenty of time . Kop