CHR15, why not post you postcode if you do not want to show your CORGI number. From what you have written, it does suggest you are not a RGI or if you are, skill level is a little low.
Pipe sizing like tightness testing is basis and simple.
I have to admit Kevin and other regulars here have 'helped' many posters. What started as idle banter has struck a nerve and now IBMs are being launched.
If you as a non RGI want to dabble with gas, do feel free. Many do that. But just let me know where you are workin so I can stay away.