Timing by vacuum
In an attempt to figure out the cause of the misfire and at the suggestion of the club members I hooked up a timing light to the coil lead and ran the engine to see if the misfire had any effect on the light. This would show if the issue was fuel or electrical. Eventually I managed to reproduce the misfire on the driveway and the timing light went out when they occurred. This suggested it was at least electrical and as I had replaced most of the components (rotor arm, condenser, distributor cap, leads, spark plugs) I decided the way forward was to replace the whole distributor with a new one and see what happened.
With a new distributor comes retiming and whilst I'd normally use a test lamp I thought I'd try a vacuum gauge this time. The idea is to connect the gauge up to the manifold and advance the distributor until the vacuum stops increasing and then back it off 2-3 inches Hg. This seemed to work really well and gave about 17Ins Hg