Silver Greyhounds are, in this electronic age, few and far between and, to the best of my knowledge, former moderately senior officers who have rendered significant service whilst in uniform. If you don't meet the template, I suggest you might want to consider TNT, UPS and so on.
Defence Courier Service may fit the ticket as well skintboymike. Rickshaw: managed 27 years in green and ended up at a reeasonable rank so seem to fit some criteria! Time spent in means I'm no spring chicken but neither was the guy I met (see my previous posting!)
DCS has massively downgraded it's entry criteria, it used to be ex WO's only - when I left there a couple of years ago an ex Sigs Sgt had been recently employed, as well as a few civilians. Being in date with DV would also massively aid your cause.
A few of the DCS guys tried to get into the QMS during my time there, but to no avail - apparently it was all secret handshakes, favours for mates, etc. You had to pretty much be in the loop already to be in with a shout. Good luck either way.
A good friend of mine was a QM until recently - he told me that the QMs, as globe-trotting messengers, are a spent force and are now being mostly replaced by casual couriers from within the FCO's ranks. The Silver Greyhound status, I was told, was originally created by Victoria. The FCO, which disliked "attached" staff of this sort, tried to disband them some years ago but their plan was foiled when a "well-placed" member of staff informed the Palace what was going on.