Right, the answers from the 34A Group on Facebook is that DOO was not agreed until well into the 1980s after the flexible rostering dispute. After that date guards still had to be provided for the tunnel section. I couldn't find out when guards were no longer required in the tunnel section.
The Northern city line was built to main-line gauge for intended use by G.N main-line trains. The tunnels are 16 feet diameter and do not restrict use of normal sized stock. Roythebus's loco hit "something dangling down" so was not necessarily out of gauge for these tunnels. The "D C electric line tunnels near Primrose hill do not allow OHLE but have actually been used by an Intercity 125 on diversion about 10 years ago (article I think in Modern Railways