locating earth points in td 90 (1987)

I'm in the process of trying to eliminate the reasons as to why the TD 90 (19j) won't start.

I'm guessing the solenoid, start motor or a bad earth.

So far, I've checked and cleaned the earth point that's bolted to the gearbox area. I assume that there are others (near headlights and under the chassis?). I am going to (as suggested) run a lead from the negative on the battery to the engine mount to see if there is an issue here. I could do with knowing if and where the other earth points are. :doh:

Thanks