Colin,
sounds like you have an old 5MGE like mine (no check connector - vac adv on distributor).
I'd recommend using a timing light.
According to the owners manual that came with mine (originally sold to the European market in 1982) -
a) Timing = 8 degrees btdc @1000 rpm max
b) Fuel required = '98 RON(Research Octane Number) or higher'
I set mine for 12 degrees without any problems (only using BP 98 though).
The real check for an old 5MGE is the alternator belts. If it is a pair of conventional belts running the alternator it is an old 5MGE. If it is a multi-v belt running the alternator it is one of the newer ones.
We'll have to drag you along to one of the cruises being organised in Perth in the next few months so we can compare 5MGEs.