This scam has been around so long the 'inspectors' are probably the grandchildren of the original scam merchants who called on me nearly 30 years ago.
And believe me they do not just 'target' expats, they try it on with everyone and intimidate people into letting them into their homes.
I liken this scam to the one in UK when a 'workman' would call and tell you that your roof has some roof slates/tiles loose and he can fix them for £20, then tells you that it is much worse than he thought and you end up forking out hundreds of pounds.
Anyway the best method as previously indicated is to tell them to 'go away' and do not let them into your house.
If you have any doubts contact your supplier and arrange with them to inspect and renew if required and issue the certificate.
We used to sell pipe and clip kits for under 5.00 euros, very easy to fit ...but don't use a lighter or match to test the joints.

, just squirt some washing up liquid around the joint-seal, if it bubbles then it has a leak ...
My Wife has just informed me that she thinks that I've got a slate loose....
Regards,
Gordon.