Worn cog.

10°C, light sunshine and W breeze.

I rode the Paddy Wagon with MapMyRide+! Distance: 71.56km, time: 03:03:45, pace: 2:34min/km, speed: 23.37km/h.

http://mapmyride.com/workout/2058361796​

Bannister Hill was okay. Actually, there was a race going up there but they let me through. The climb was not too hard but it may have led to mechanical problems later.
I noticed the odd cracking sound in the following miles. On wtgd approach to the next village, the chain jumped off the rear cog and was trapped against the spokes. At least I could stop safely to fix it.
After fixing it, it looked like the cog’s teeth are narrow and hooked. They’re worn out.
A single-speed bike does not share out the wear across 8 or more cogs so you should expect the gear to wear faster than multi-gears. Considering the age of the gear, this is good going.
By the way, the ride was nice. It closed over later and the wind picked up. But, spring is here, daffodils in full bloom.