The ECAC agreement theoretically allows permit amateur-built aircraft to operate throughout Europe.
The snag is that each country has its own interpretation of the agreement.
France, Germany, Holland, Denmark, Sweden, Italy have a fixed arrangement not requiring any prior authorisation (last I knew). Spain, Portugal, Belgium and Channel Islands require prior permission (and the Belgians want their palms greased too). The UK also requires prior permission to non-UK homebuilts (see AWN52). So much for European harmony.
The LAA have a leaflet, which isn't very upto date so a bit of research may be needed.
In practice, it's not too onerous