I did the C2C stretch from Sunderland to Stanhope and back the other day.
Except I didn't because the path from Stanhope to Parkhead Station is poor and difficult to follow - the woman in the cafe at Parkhead confirmed this.
So my suggestion would be to use the B road from Stanhope to Parkhead, taking in (reluctantly) Crawleyside Bank.
Again according to the woman in the cafe, lots of riders opt to do that.