I made some progress today.

I replaced some fuses and the headboard works again.
The variable targets still have problems, so I looked through the schematics and thought it was to do with the I Relay.
Turns out the I relay has nothing to do with the vari-targets, but it has a hell of a lot to do with moving the football up the playfield.
The football often doesn't move up the field as it is supposed to, and it frequently sticks on the try line.

I had to slightly adjust the position of a cam on the I relay and the football moved up the field perfectly exactly as it is supposed to.  Unfortunately it only worked a for about 5  minutes, now it is stuck again.  Will have another look tomorrow.

cheers
Mark