At the end of the day, the MoD has no real power to stop anyone writing a book about their time in the forces and making money from it.
Looking for Trouble
Bravo Two Zero
Tornado Down
Sea Harrier over the Falklands
etc etc.
IMHO, the real issue is the MoD believing that it has the right to stop these people telling their stories. I don't think the Official Secrets Act would cover any aspect of the Royal Marine's and Mateloe's time in Iranian hands.
Let them earn a few quid. Gordon Brown is not exactly throwing money at us these days so, fair play to them.