Phil, echo what Bob says re BW east of Tan Hill....
But what I’ve done (a few times) as part of a loop (starting west of Gunnerside cos it’s easy parking) then heading north to reach Tan Hill, is to take the road east of Tan Hill for about 3 miles then turn R into William Gill (rideable) to join the BW...
Some good technical riding follows, careful navigation required, (but I wouldn’t bother unless it’s dry and has been for a few days) eventually reaching Great Punchard Gill
Then BW south for 2.5 k to reach the track just before Moor Ho (OS 1:50000) - turn R and head for the infamous Blakethwaite Gill *** - keep going almost to where the Gill finishes, then drop into it.... ermmmm
A lot of the BW in Blakethwaite Gill is very good technical riding but don’t fall off (in the wrong place

) - after about 0.5k you can cross the stream and reach Gunnerside Gill... ST and a fall here is very serious cos at one point you could tumble down 40 or 50 feet to drop a steep 15 or 20 feet into the stream landing on killer rocks - ouch
Then plain sailing back towards Gunnerside
( from the start I would usually head north, east of the River Swale, taking the BW towards Tan Hill - some good technical riding... the climb from E Stonesdale to Shot Lathe is “challenging” if you manage it without a dab or a stop)
***
I came off once on that section - a bird’s eye view of my front tyre almost in slow motion as it came off the track just by an inch... an inch is too much and the bike slipped to the left and I came off, somersaulting twice ! On my second somersault I saw the bike flying past me, stuck a hand out, and miraculously caught the frame with one hand...
Very lucky for me too that after my second somersault I came to a stop...
I wasn’t injured a lot at the time but in the middle of the night I was in a lot of pain in my left leg and could hardly walk...
( so if anybody’s daft enough to try all this please be very very careful...)