Been out today and done Haws Clough Lane (Road of Death mentioned by DanBo), and carried on to Running Hill Lane, looped round clockwise and back down to then do Chew Road (suggested by Parrot of Doom).
Haws Clough Lane seemed to be about 15%, and some of the bits of Running Hill Lane were a bit steeper. Haws has a long view of the steepness ahead, unusual in these little pennine lanes. Lovely moorland farm views from the top.
Chew Road was a different beast altogether, especially on a road bike. The steepest bits are tarmaced, and are around 20%, but the hardest bits are the gravel sections that are nearly as steep, especially the second to last one where the gravel is small and deep. I found it really hard to maintain traction and momentum without the front wheel squirrelling around too much. nearly lost it but luck found me a bit of surface under the gravel and I was able to stabilise and carry on without stopping. I cycled up back into sunlight and was treated to the view from the top with a huge blood red setting sun.
Going back down was a bit tiresome, as all the drainage ledges meant constant slowing right down and speeding up a bit again. Going down on the gravel was OK, just relax, allow the bike to find it's own way, and plan your line firther ahead. Oh and braking only worked if heavily modulated.
Don't think I'd do it again on a road bike, but glad I've ticked it off.