A couple of revisions you may consider
A66 avoidance route from Barney – head East to Whorlton, cross Tees by the suspension brige, then back road it to Thorpe Farm on the A66, underpass and bridleway to Greta Bridge, then Barningham and back road it via Scargill.
Newbiggin – you've missed the double chervon and stuck on the B6277, although not double chevron the Hude is steep enough to be worth inclusion and descent down Milly Lane (the double chevron) worth doing. Good views if you can take your eyes off the road.
Cotherston to Middleton – Turn off to Hunderthwaite (Mile 46 in Darlington Start), go over Bail Hill into Mickleton. Then as Darlington Route to Middleton. You could also turn up to Briscoe from from Cotherston, cross over infront of the Reservoir to Hury. Over past Botany (reverse miles 52 to 49 in Darlington Start), up Bail Hill, left at the mast and down in Romaldkirk, over to Eggleston, B6282 to Middleton.
It seems a shame to miss out St. John's – Langdon Beck – Garrigill – Nenthead – Cowshill
If this is next spring summer I'm interested