That, is fudging awesome! i'm sure there's a few of these around, but i've never seen one as complete as this!
There's a rumour that there's a similar bunker (but many times larger) near the Alford ROC post, but i can't track down it's entrance so it could be hear say.
Stunning dude, great report.
I think you're on about Skendleby dude, not far from Skegness. It's now in use for servers by a company called Centrinet.