My girlfriend might be a bigger problem though, she isn't keen on the idea of me joining but I think it's something we can get through.
Don't underestimate the importance of this issue. Going the dip corp is a big, big decision that has huge implications for relationships. Your work doesn't end at 5.30. You're effectively on call for all of your time abroad.
I had some exposure to the service some years ago and what I noticed is how the demands remain the same despite changes in personal circumstances. It's a tough, tough life and one that's conducive to only rare circumstances. Moving around the the world every couple of years might seem like a hoot in your early 20s but it becomes more of a pain when you want to get married, have kids, etc. Have you considered, for example, how you girlfriend might react to being expected to host dinner parties in your home overseas for colleagues from other countries? If she's not keen now.......
IMHO, and based on my albeit limited exposure, diplomatic service only works, over a lifetime, where there is a docile, dutiful wife who is happy to up sticks every couple of years, tend to the kids, host work parties, etc, etc in exchange for being invited to occasional Ferrero Rocher reception herself.
This might have been possible where a single salary was enough to buy a house, cover bills, etc but where both partners are working, the pressure is enormous. The anonymous diplomat might have some insight, for exampke, on the incidence of marriage breakup within the service, but anecdotal evidence I've heard suggests it's proprotionally higher than other parts of the civil service. (I'm open to correction on this one, though).
You're making a huge lifestyle choice here. Look beyond the "See the World" brochure.