I wondered what Pennine Paul was on about because what you originally described didn't have a descent into Hebden Bridge, but then I checked the map you linked to ...
Mythom Steeps! Er, yes, you really
don't want to organise a ride going
down that! In dry conditions, with good brakes and no traffic it is safe but you have to be careful. With poor judgement, brakes less than 100%, greasy roads or oncoming traffic it can be very dangerous indeed.
I was in a car involved in a multiple pile-up on that descent! All the drivers were driving too fast. A mate was giving me a lift back from work and was too close to the guy in front who braked suddenly at the bottom of the really steep bit when a car cut the corner coming the other way. The car in front slid into the one coming the other way. We slid into him, and then we got rammed from behind - all drivers equally to blame.
I have toyed with the idea of inflicting it as a climb on a forum ride, but I'm not fit enough to climb it at the moment so that's a pleasure for the future!
I suggest that you follow the road round from Blackshaw Head through Colden to Slack, then descend through the woods and the lower part of the Heptonstall Road as we did on the Trawden ride on Sunday.
SHAUN - check your routes against OS maps!
That routes website displays ancient rights of way as if they are all roads - they aren't! Your route 'does a Sat Nav' up some footpaths from Walk Mill. You need to go up Greencliffe Lane!
I'm not sure if I'm free at the weekend because my sister might be up. If I am free, I don't fancy the Rossendale valley much so I might make my own way up to the Manchester Road lights via your route in reverse so I am bound to meet up with you and ride back up the Long Causeway and then up the Cragg Vale climb with you.
I'd then either ride back - Littleborough, Calderbook, Walsden, Lumbutts, Mankinholes, Hebden Bridge OR go the other way - Blue Ball Lane, Hubberton Green, Boulderclough, Mytholmroyd, Hebden Bridge.
I'll let you know if I can make it. Roughly what time would you expect to get to the Manchester Road traffic lights?