Solved!
Along with an engine rebuild (at my request) we established the INITIAL reason for misfire. The shaft assembly inside the distributor was loose, you could literally rattle it by hand. Welded it up, drilled it through and got a tight fit again. Also found a piece of plastic inside by the advance plates which could have been stopping them from opening.
A good tune up and she runs bloody SWEET!
Engine has been rebored, piston rings fit, shaved head, ported head and rebuild with new gasket, valves, bearings etc.
Also fit a new clutch as the dude before me put it in backwards lol? It had worn down so much the rings were literally about to break off.