HELP-NO SPARK!

I am hoping someone out there may be able to cast some light on what might be causing my pickups lack of spark.
It is a 1987 4x4 pickup.
Symptoms prior to complete fail were slight misfire from cold which did clear, thought at that point that it might have been simply a damp issue.
Next episode resulted in engine failing to start following a steep climb through woods loaded with logs. Exhausted the battery trying to re-start, gave up and returned perhaps 15 minutes later by which time the battery had recovered sufficiently and started the engine on the first turn of the key.
Drove home (4 or 5 miles) with the load of logs, stopped the engine and it has refused to start since.
No spark at the plugs and not really sure where to start looking, hence the plea for suggestions!
The initial misfire would now seem to indicate something was breaking down, rather than a damp issue as I first thought.
OEM style coil together with what looks like a resistor plus a small guarded unit (with heatsink) mounted on chassis rail under passenger seat (RHD).
Have seen mention of cam/crank sensor on other posts but for the life of me can't find either!
Should mention that its a carb engine.

Thank you for reading my essay.