I've not done Great Dun Fell either but that's mainly because I rarely take my bike on holiday if I'm the area. Plenty of running/walking to be getting on with you know!

It does have a good reputation and no doubt the closed road is an added attraction. It is also one of the few UK climbs that could claim to be anything like you would find in the Alps. That said, I'm not so sure it is the toughest climb in the country. There are a few that are longer and plenty that are much steeper. The Strava segment lists it as 7.1km with an average gradient of 9% and maxing out at 20%.

I've long been a fan of Simon Warrens cycle climb books and I've ticked off quite a few mainly in Wales and the Midlands. Most of his rating I'd agree with and GDF gets an 11/10 but so does Bealach-na-Ba which he describes as 'the Holy Grail, the toughest and wildest climb in Britain.'