If you are doing a wet pressure test, that implies (probably) that the
pipework is all complete. If you have a sealed system, you have the
system's pressure guage anyway. If you have a vented system, you can't
do a pressure test (unless you seal off the pipework to the header
tank). So I can't really picture how you would do a wet pressure test
using a dry pressure test guage in any case.

Sorry, probably not explained it very well.

For testing a sealed system, I was thinking of performing a wet
pressure test.

I was originally under the impression that I could use the Screwfix
gauge, until I saw it was a "dry pressure" test tool.

I was therefore wondering if I could still use it, once the system is
filled with water, by using the Schrader value to increase the
pressure - with air.
I.e pressurise the water in the system by pumping air into it.

I suppose the pressure in a wet pressure test *should* really be
provided by either a) mains water pressure or b) a hydraulic pump.

I'm really wondering if I can buy the Screwfix tool to perform wet and
dry testing, or at least use the gauge to indicate the pressure during
a wet test.
(since I may not have the boiler connected at that point).

Anyone, any ideas?