There has been talk recently and in the not too distant past about the 20/20a and X20 and reliability.
Apparently the scholars working impacts on the 1610 X20 from Park Lane.

Recently, I have observed the 1710 X20, also running late.
The last two nights, it has been at the Carville traffic lights (outside the P&R), at 1750.
The timetable 'should' mean it gets out of Durham on time at 1809 (20a), but it is leaving it very tight.

I often use this particular bus, as it should give me plenty time in walking up to Durham station for the 1825 EC to Glasgow.
Seriously considering getting the earlier bus next time.

No idea what the issue is, but guessing the Solar is late, due to traffic/delays on a previous run.