My normally good Excel won't start

I have the Service Notes & wonder if the Start Relay is failing.
Where is the Start Relay physically located on an early car (old dash with fuse box near right ankle)? Suspect it's on pedal box or above fuse box - but I haven't found a pic of the old setup in my Service Notes - only a pic of the new setup.
I have had two different starter motors on it & the same result. The trouble seems the behaviour of the solenoid (on the side) rather than the motor itself.
When engaged it doesn't go in properly and "click - click - clicks".
This is almost certainly a lack of current under start conditions. A possible problem is the earth, but this does measure as very few ohms of resistance. [But this test won't show a cracked heavy duty cable.]
I've tested by bypassing the ignition sw. with a shorting link. Sometimes it starts, sometimes it doesn't. I plan to do more testing on this today.
Although I have more work to do, I thought of posting this now as one of you might have solved this before.
fingers crossed - TIA.
MickW.