Proceedure in pressurising expansion vessel in combination boiler.

I have a Worcester Heatslave 9.24 RSF combination boiler to heat my one floor three bedroon flat.
I have had to drain and replace a diverter valve.
The expansion vessel should be prepressured to 0.5 and would like to check this but have some questions at what stage should the 0.5 bar be read.
Q1. What is the correct proceedure in pressurising the expansion vessel. Before the system is pressurised with water or after, or does it matter.
Q2. Will the expansion vessel pressure reading be different if the system is filled and pressured with water. ie. if the system pressure is 1.2 bar should the pressure at the expansion vessel read 1.7 bar (0.5 expansion pressure + 1.2 system pressure) OR will the expanison vessel read 0.5 regardless of the system pressure?
Chris...
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Chris Wilkins wrote:

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Ed Sirett - Property maintainer and registered gas fitter.
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