Grabbers,
I made the jump at the 22year point (3 years ago). No major problems, you soon get back into the classroom learning routine. Also your previous experience is a big help in the simulator, whether it is just being at ease on the R/T or having an instinctive reaction to pop up traffic when giving a RAS. The courses aren't easy and you have to fully commit throughout the 18 months to 3 years training but with a Mil background you'll be used to that! If you are thinking about NATS, they are quite big on recruiting experienced controllers at the moment so give their HR people a ring.
Regards, ADIS