The European airspace documentation can be found here:
ECAC RVSM Library
In particular the MEL is included in TGL6 and the Flight Crew information pack.
The MEL is:
AIRCRAFT EQUIPMENT
The minimum equipment list (MEL) fulfilling the MASPS consists of : (see JAA TGL6)
1.Two independent altitude measurement systems each equipped with:
2.One SSR transponder with an altitude reporting system in use for altitude keeping.
3. An altitude alerting system.
4. An automatic altitude control system.
Unfortunately the ATC Manual is not available on the site as it is undergoing a rewrite - so can't see what official guidance is given if a transponder fails.
