At present you need a valid multi-engine IR(A) if you are starting your first multi-pilot Type Rating course (FCL.720.A (d) - see CAP 804, Section 4, Part H, Subpart 1, page 2). This may change in the future (EASA NPA 2014-29(A)).
Interesting, found the change on page 64:
Multi-pilot aeroplanes. An applicant for the first type rating course for a multi-pilot aeroplane shall be a student pilot currently undergoing training on an MPL training course or comply with the following
requirements:
(1) have at least 70 hours of flight experience as PIC on aeroplanes;
(2) hold or have held a multi-engine IR(A);
...
Any idea when this change is going to come in to force?