There's little can be done but I'd ask to be shown the valve & the test it has failed and mention that you were not aware the DIN outlet was tested under BS EN 1802 only the cyl-valve fit, better still if you know the thread & can refer to the cyl-valve thread by size, I'd even ask to see where it says the outlet needs testing in the procedure, & make it clear you think they have caused any damage to make it fail.

If they think you know your stuff they're not going to try it on

Luckily I trust the chap I take my gear to & he knows I used to work on HP gas systems & cylinders.