1800ES Electrics & Ignition

[bells]

OK, so this will be a bit of a long post as looking for help with electrics. My P1800ES, over the course of a mile or so went from driving perfectly to what sounded like misfiring and then completely conked out.
After checking all the HT leads I moved to the LT system. All normal tests ran fine until testing the voltage from the -ve of the coil to earth with the points closed and ignition on. Result - battery voltage. Immediately think perhaps the points aren't closing properly but under same conditions all parts of contact breaker and LT contact to distributor are reading zero voltage to earth. So there's some issue (resistance) between the coil and the distributor in the LT circuit thinks I.... I know that the tacho is in between those two from the wiring diagram, and when I run the resistance meter across the unplugged LT lead from the coil to the distributor indeed it seems to have infinite resistance. Bingo.
So to see if I can fix it I attach the coil (-ve) direct to the distributor (primary terminal) - thus bypassing the tacho, re-attach all the HT system and does she start? Does she f**k! Exactly the same sound on turning - loads of starter motor - no ignition.
So now I'm left that I think I've got a problem with my tacho circuit, but that bypassing it doesn't get rid of the problem with the ignition.... agghhh. Any ideas? It's my first time with any of this so running low on ideas myself.