Looking at starting from the car park at Haweswater, doing Nan Bield, down toward Kentmere and back up via Gatesgarth Pass.
Looking at the map, there’s a track breaking off to the right of Gatesgarth towards Mosedale, then Swindale Head and back to Haweswater via the Old Corpse Road. Is this extra loop rideable/worth riding, or am I better just going straight up Gatesgarth?

Personally I wouldn’t bother with the Mosedale loop. The Old Corpse road is good but i don’t think its worth the slog to get to it.
if you like technical riding and are up for a challange I would do Nan Beild as your planning and go up Gatesgarth on the way back. When you get to the top of Gatesgarth do a left up onto Harter Fell and go back down Nan Bield to Haweswater.
I have to admit Nan Bield down to Haweswater is one of my favourite UK trails. It’s an absolute brut
I might have a Strava route of the above that I could GPX for you if its of interest
Cheers
Mark

That’s what I’d do too. You get to walk it up past Small Water and see what you are in for. If you are up for it after the ride you could take the left up Harter, or just carry on down a fun but straight forward blast down to Haweswater directly.

Heading up there with a few mates the week-end after next. That sounds like a good loop. Looks reasonably straightforward on the map but scarey carey have sent you an email if you wouldn’t mind sharing the gpx.

Thanks for the advice, folks

http://pedalnorth.com/content/nan-bield-and-garburn
http://www.pedalnorth.com/content/eastern-passes

OP- I did the exact route you described back in April. Depends what you want. The bit from gatescarth to mosedale is boggy but rideable for 75% depending on the recent weather. From the cottage to the top of swinedale is good single track and the drop into Swinedale itself was a hoot. It’s hike a bike up the old corpse road, ride across the top and the descent is pretty awesome. Like I say, it depends what you want. It’s a very isolated place, you won’t see many other folks and the views are stunning which I enjoyed but it’s a slog to the top of the old corpse road. A word of warning – storm Desmond caused some major land slips between small water and the top of nan Bield – for large sections, the path has just plain disappeared – was ok to hike up, dunno what it would be like to ride down … It may have improved since Easter but I doubt it. Sure you’ll have fun whichever route you take.
link is to the view from the top of the old corpse road
https://flic.kr/p/FGd8Ue

https://flic.kr/p/FGd8Ue