…saying that he should have had stuff like MEL…
A MEL is not a requirement for a private flight in Germany!
I had the same thing in Hof, aircraft papers and license.
We had quite a lot of ramp checks in Le Bourget – as the former president’s girl friend (now his wife) used to fly with us.
Times are changing – now S. got " ramp checked " himself … 
For a recent ramp check in the UK all I was asked to show was the aircraft tech log.
So what paperwork is it MANDATORY to carry according to the law? Of course the law is probably not very clear and do we go with our own CAA rules or the country of arrival or is it mandated by EASA for example?
As always, a whole set of law texts apply. First of all Part NCO GEN 135 for non commercial operators.
The interesting bit of GEN 135 is this:
Any other documents that may be pertinent to the flight or required by the States
concerned with the flight may include, for example, forms to comply with reporting
requirements.
STATES CONCERNED WITH THE FLIGHT
The States concerned are those of origin, transit, overflight and destination of the flight.
So make sure to research the national law of all 30 countries you might overfly… In reality study the requirements of your country of registration. I have every document I ever got as PDF on my phone. This is easy to do nowadays so why not do it? I don’t carry all documents but the minimum required by German law because in case of a check, I don’t want to present any papers that I’m not asked for. My natural reaction to a government official demanding something from me is “on what legal grounds am I required to present this?”…
For a recent ramp check in the UK all I was asked to show was the aircraft tech log.
I’m certain that carrying the tech log can’t be a requirement in the UK. It’s far too valuable logbook to carry with me at all times, I keep ours safely at home. We have a log we keep in the aircraft that I transcribe to the tech log when I get back home. (Or is it because you’re a non-G-reg visitor?)
No such thing as a tech log in n-reg. The aircraft logbooks would never be carried. There is a journey log requirement but that is different.
carrying the tech log can’t be a requirement in the UK
AFAIK in switzerland it’s even forbidden to carry the techlogs (we have 3, propeller, engine, and airframe) in the aircraft, you’re supposed to send them separately.
