I found the beast of all hills on Saturday morning. I went over to Llangollen and did the Horseshoe Pass and then headed out of llangollen towards Glyn Ceriog. It is here:
http://app.strava.com/segments/1836553. Strava says parts are 67% which seems a bit OTT, however it is definitely the steepest I have ever done and far steeper than any part of the Horseshoe Pass. I had to push part of it when a tractor came steaming downhill and I had to pull over and stop and then couldn't clip back in. Riding down into Glyn Ceriog was also terrifying! I recommend it to all.