I checked LASORs to find the definitive answer.
The reference is on
http://www.caa.co.uk/docs/175/SECTION%20F.pdf
Scroll down to F4.1
The section is entitled
Pre-requisite conditions for training
An applicant for the first type rating course for a MPA shall provide evidence that the following requirements have been met:-
a. have completed at least 100 hours as pilot-in-command of aeroplanes;
b. hold a current and valid multi-engine Instrument Rating (Aeroplanes)
With all due respect to other posters, I hope this settles the point; it is only of importance to those new joiners
without a previous multi-pilot type on their licences , which I had not previously appreciated.
Edited for clarity