Dunno. FPNs are always a conditional offer, but I can't imagine why anyone with a clean license would have the FPN revoked so they could be dragged in front of the magistrates instead. If they found you'd got a license like a snooker score I could understand it, but since yours is clean it seems a bit excessive. I thought a trip to court was for when they catch someone going really fast (30mph over the limit I think), but then why give you the FPN in the first place. The copper would have just said he's reporting you for the offence yadda yadda yadda. Plus 21 and a bit mph over still sounds like FPN territory to me. I got clocked at 25 or 26 over and only got FPN for 3 points and 40 quid fine, though that was about 8 years ago so they might have moved the goalposts since.
What seems particularly unfair is that you paid up, then they sat on your money for a whole two months, and then they returned it with no explanation other than saying the ticket had been cancelled. Then nothing for over six weeks before a summons shows up. To put it mildly, they're taking the wee-wee. First stop
http://www.pepipoo.com. Probably won't hurt to phone plod and ask them for an explanation. Then I'd go and find a solicitor who can tell you if they're actually allowed to jerk you around like this, or if there's some cut-off time limit they might have exceeded.