On 16/02/2018 19:02, Jim.GM4DHJ ... wrote:
Is it legal for me to use a monometer to check the gas pressure at the meter
and at the boiler ?

If you are a owner occupier, ie not a landlord, then it is not illegal.

Good working practice says you should check after replacing the 'screw'
blanking using leak-spray.

If you are checking leak rates then there are some procedures you need
to go through to get a true value.