Digsy wrote:

Ok, the scenario:

My central heating oil burner oil pump is leaking - not a huge amount,
but it needs replacing. This I can do for a cool ?40-ish, but the pump
pressure needs to be set after it is installed.

The pump pressure depends on the oil burner nozzle size.

The nozzle size depends on how the burner was set up to heat my house.

I would simply plumb in a temporary pressure guage.
A T, connected up to a tyre pressure guage will work fine.

Alternatively, if you can observe the flame, and the boiler is not a
modulating sort, then simply taking a photo beforehand, and turning it
up till you get an identical picture is the same.