(29 Dec 2019, 11:50 am)big mac wrote Yes it was 10 minutes between Newcastle and Durham, as they also increased the frequency of the then 24 from Wrekenton to Newcastle to every 10 minutes. 
This created a combined every 5 minute frequency between Low Fell and Newcastle, but the problem was that many passengers along the Durham Road corridor use the services south of Low Fell where the 24 turned off to go to Wrekenton, so they lost out, and the 21 struggled to cope  with the strain put it on it south of Low Fell.

In theory, I think it was a half decent idea - but not carried out in the correct manner. 
The idea of turning a 24 every 10mins at Wrekenton wasn't one that filled me with much confidence, especially as you say - the strain was put elsewhere on the route. 
The 24 carried fresh air between Harlow Green and Wrekenton and the 21 struggled the rest of the way. 
Something to help beyond Harlow Green, rather than 'just up to', may have worked a little better imo.