Bandook,
Mate im ex Royal (LC) getting your foot in the door is always hard but keep at it and something will come of your hard work.
If i was you limit yourself to a few companys at a time when emailing as alot of them all at some point do different things within the MARSEC industry from transits, coxswain work or even Security watch officers on board security vessels or client vessels.
After you have sent an email i would say always ring them to make sure they got the email a few days later nothing much just a confirmation, your name on there thoughts after a conversation with you may land you a job (proven time and time again).
Your an ex royal marine so this always helps, i have nothing against Securwest and have not worked for them BUT there reputation is not the greatest amongest the consultants, I'm not saying dont ring just be abit choosy about the companys you work for some are not great for money, and some leave you out on a branch.
Drum-Cussac, IMSA, Libertine, MUSC are some good companys mate to start with and all do a varied range of jobs for all skills.
Dont bullshit on your CV, and dont try to dazzle the companys, again what you have done in the past (ex Marine) stands for alot and you already have 2 qualifications that will hold you in good stead.
Minimal Risk are doing a maritime job at present and they have a high turn over of lads (gain some maritime experiance then leave) its not the best paid but it does have all the gear you need and a few of the lads (TL) have a wealth of knowledge give them a try to get that foot in the door?!?!?!
More then happy to pass on alittle more info refs qualifications and companys the rest as they say is up to you!!!!!