The decker I saw heading to Seaham from Houghton at 8.50 cant have been the 877 then. Wrong direction, wrong place etc.

Were still no closer to solving this one haha. Lets just pretend and forget I ever saw the mystery decker :p

As for Deptford working the 865, the 20 you mentioned would find it very tight at that time to get upto Fram and turned around in 13mins - theres not much leeway espec as it will be coming across school and commuter traffic from more or less starting off at Park Lane. It would need to get into Durham on time and the 20s dont tend to do that...
At that sort of time it has taken me anything from 5mins to 45mins to get down Gilesgate and upto Fram in the car and that hasnt included having to head off into the bus station. It would then need to turn around somewhere and I cant see how it would do that at one of the roundabouts up there quickly, which would mean going upto the Arnison Centre, which creates its own problems at Newton Hall.

On another note, a Loylne 3865 is parked up just outside the Arnison Centre in a bus stop, appearing broken down (engine off, destination off, no passengers and the driver sitting on the rear seats with his feet up reading a paper)