The fact that you are already a CI, and therefore 'on the books' and hold the relevet security clearance, makes the process a lot simplier. As a posting on a VGS is an extra duty, you wouldn't need to appear on their establishment.
As has been stated, arrange to speak with the VGS CO or CFI, who will guide you through the process. I don't recall a conversion proceedure, as civilian ratings didn't carry any 'points' in the CGI training process. You'd still be required to complete the required number of flights to pass each stage of training, although passing should be much easier!
When I was active on a VGS, a CGI candidate who could already fly, was snapped up before they changed their mind...
Edit: I'd be surprised if Flt. Lt. Mockeridge was OC 612, as OC VGS is a Sqn Ldr post, but I'm sure he'll be able to help.