A vacuum gauge would be needed, it would also be helpful when setting the timing (and they don't cost very much). Yes, it could be the vacuum capsule (British term for advance mechanism) in the distributor, tested with a vacuum pump (or by observation, it should move when under vacuum). There's also something called the "RAVE" manual available for free on line that is real helpful in tracing problems or fixing things. It's a digital repair manual.

It's a shame the Rover P6 series wasn't appreciated, it's much better than the same year (or any 60's or 70's) Benz.