Hi Richard
With your battery in the 'boot' - the length of cables involved will make volt drop problems the more likely.
Relocating the battery to 'under the bonnet' would probably overcome your problem, without actually find the specific cause.
As Steve says - running a separate cable may be bypassing the cause, but it hard to see what there in circuit that would get hot enough to produce a higher resistance of any significance to be worth bypassing.
It may be that the bypass cable is just helping to reduce the voltdrop at the coil while cranking takes place. Which is exactly what the booster battery would be doing.
The fact that hot starting with the bypass cable only works sometimes while the booster battery always works tends to show how marginal the problem is.

I still have a 'feeling in my water' that the coil may be the problem.
I know you have already replaced this, and that youhave no ballast resistor.
When you have a hot start problem - does the coil feel very hot to the touch ? If it does then this may be the source of your high resistance and would suggest that a ballast resistor may be required.
It might be interesting, if you still have your old coil, to very quickly reconnect this old 'cold' coil and see if you get a successful hot start.

All the best with this knotty problem
regards - Chris