As a fen-man born and bred but now in exile, it was always my understanding from the locals that the name came from the fen reptiles, specifically the newt - having a yellow underside, but I would certainly defer to other definitions if a definitive one came along. So, IMO you have to be from the flat bit at the bottom, and not the mountains N of Horncastle!
Having said that, I'm up for some rides with any Lincolnshiremen (there aren't that many of us anyway), particularly in the flat bit to get used to my new 700c recumbent. Tetford Hill would be OK (got up that on my 20/26 'bent), but Belchford may need a bit more practice.
Will keep an eye open for news of rides. And what is this FB of which you speak?