Opus electronic ignition unit that fits in the distributor. What do you mean when you say "it won't turn over", do you mean that nothing happens when you turn the ignition key or that the starter motor turns the engine but the engine doesn't fire up. "Turn over" would appear to have different meanings depending on location
Sorry, missed your previous reply. OK, nothing happens when you turn the key. Sounds like a wasted battery or bad battery connections or bad starter motor connections or bad engine earth straps. If there's a battery cut-off switch that could be faulty too.
I've just remembered why I suggested worn brushes. The same thing happened to me with my ex-Silver Spirit. After I had called the AA, because they were extra busy on a holiday weekend the car sat cooling for an hour. It started Ok and I drove home. The problem was worn starter-motor brushes.
Without the actual chassis number it's impossible to know whether this car has the old points-type ignition versus the Lucas Opus electronic ignition, since it was introduced during the 1975 model year. The SWB cars had the Lucas Opus ignition starting with sequence number 22118 and LWB starting with sequence number 22673. But even though the Opus ignition tends to just crap out once hot, there are other symptoms, and the starter still tends to crank even if the engine will not turn over until the ignition cools down and starts working again (for a limited time). For my experience with a failing Lucas Opus ignition, see: A Christmas FTP!! . . . on the Australian forums.
You have a points ignition. I strongly suggest your capacitor has failed. I would replace the coil, rotor, cap, and capacitor if this has not been done in the last 5 yrs or so.
Opus ignition arrives after your chassis at 22,672
In my case, ensure wiring and connection around startemotor are clean and tight! Replace earth cable at battery to new. Often relay switches cannot get rid of their electric charge if earth cable are old. Note, electricity flowes over surface, green corroded earth cable is a sign of electrical flow handicap made worst with heat.