I recently had a cylinder tested (passed). The Inspection and Test Certificate has a customer signed statement that the cylinder and valve will be tested in accordance with EN 1802, EN 1968 or BS 5430 and should either the cylinder OR valve fail THEY will be destroyed and or not returned to the customer.

The way it's written seems to give them licence to scrap both even if either one fails! That would seem to be a liberty, as it forces the customer to stick with them for whichever part requires replacement.

However, if you were to take the cylinder elsewhere you would end up with two bills for testing plus replacement parts. So yes, short and curlies comes to mind.

Ask them to demo the no go gauge failure when you next go in.

If it is the main air outlet (DIN connection), it's huge with a course thread, as you say does not need much to compress an o-ring seal, no need to apply gorrilla force on the threads.