Agree, should be about 20hp. I've got an M25 which is 26hp if I recall. There is a manual available online somwhere though I can't find it at the moment. I've got a copy on my computer at home so let me know if it's any use to you.
If you click on 'Perkins' on the left hand side you can download a user manual for the Perama M series and a workshop manual which covers the M series.