My companion is disabled and we wish to complain about the poor service on a recent flight in/to/from Europe. Can you help us to contact the appropriate agency?

ENAT does not handle passenger complaints nor is ENAT an authoritative (statutory) body for travel or tourism businesses in Europe.

Air passenger issues concerning flights within the EU members states and flights in or out of the EU are governed by EU regulations. The regulations are enforced by the National Enforcement Bodies (NEB). Passengers must make their complaint, in the first instance, to the airline. If you are not satisfied with the answer you receive, you should complain to the NEB in the EU Member State where the plane departed or arrived. 

The European Commission runs the Air Transport Portal (website), where you can get further information about the EU regulations and passengers' rights.

At the above website you will find a Complaints Form which you can download and fill in. Surprisingly, the Complaints Form does not include specific reference to problems concerning boarding /seating passengers with mobility impairments, so you will have to explain such issues in detail. You should send the Complaints Form plus any supporting documents (e.g. copy of your previous letter to the Airline, flight numbers/ boarding passes/ seat numbers) to the NEB.

The European Commission website referred to above lists the NEBs for all EU Member States in a downloadable PDF document:

Updated List of National Enforcement Bodies

You can also find further references to accessibility issues in air travel at the ENAT Theme page on Airports and Air Travel.