Flying experimental (Permit) AC around Europe
 

How does it look right now? Most European countries don't need overflight permit for experimental aircraft reg in EU, right?

Assuming that the aircraft is amateur built and currently registered in Europe then it should be fairly straight-forward. Have a look at this document from the LAA which may not be fully up to date, but will give you an idea.
You should be able to find the requirements in each country's AIP in section GEN 1.3, but there is considerable variation in the information provided by different countries.

I believe Spain and Portugal still require prior permission, but most others are accepting of the general ECAC agreement.

Thanks Mark, but allow me to warn the document's info on Belgium is quite out of date. Even the contact addresses are no more valid.

Isn’t that because the news from Belgium is good, Jan?
No need to pay anymore?

So I understand, yes, but I am not sure about the exact details. Couldn't even say whether one still needs to request a permit. Must be somewhere in the AIP...

No need to request a permit. Just fly in.

Links don't seem to work, but it is a Royal Decree of 8 June 2017 modifying a Royal Decree of 16 March 2009, published in the Moniteur Belge on 21 June 2017.