I have been in a very similar situation.
My JAA license expired in September 2014 (as I'd also totally forgotten the 5 years expiry date), and I discovered this in August 2015 when preparing for my revalidation through experience.
So situation was: License itself expired, SEP rating current.
When I fessed up to one of the instructors at the flying club, his immediate comment was along the lines of "Well OK, at least you haven't crashed anything".
I filled out the necessary form to renew my license and took everything down to the CAA office at Gatwick the next day and explained the situation. They were actually very helpful - but one thing that they made very clear, and took a copy of the relevant page of my logbook, was that I could not act as PIC during the period that my license wasn't valid. I could log the dual time, but none of the command time.
A somewhat amusing aside from this tale is that the one thing that caused the CAA a degree of bureaucratic angst - and delayed them issuing a new license for 24 hours - was that nowhere had a box been ticked to say that I could speak English as a native speaker. Despite the fact that I was clearly having a fully fledged conversation with them, I had to go back to Redhill armed with a piece of paper to get a CFI to sign it to say that I can indeed speak English.
Fly for a year without a license? No problem sir. Box not ticked to say you can speak English? Major issue, please fix at once
