What was the typical hourly service pattern in the 80s and what was the haulage? Was it all DMU by the late 80s or were there still locomotive-hauled services (e.g. to Banbury and onwards?)
The Great Central Main Line closed in September 1966 and that was the end of loco hauled services at Marylebone until after privatisation, with a few occasional exceptions.
Trains between London and Banbury were through Birmingham trains to and from Paddington until the WCML electrification was completed. Marylebone-Banbury trains only started when the Paddington-Birmingham trains were withdrawn.
Marylebone had a fleet of suburban DMUs for trains to Aylesbury and High Wycombe, though a lot of the heavy maintenance was done at Bletchley.
In the early 1980s the off peak service was very sparse with hourly trains to Aylesbury and High Wycombe. On alternate hours the High Wycombe trains ran through to/from Banbury. The Princes Risborough-Aylesbury service was peak hours only, though the few trains via that route were to/from Marylebone.
This was probably the only London terminus I never visited pre-privatisation.
That's almost certainly because there wasn't really anything to see, though the same could also be said of Fenchurch Street, Cannon Street and Charing Cross.