Ben is probably right, poor connection somewhere, most likely the earth - unless the battery is earthed to the engine or gearbox suspect the engine to chassis earth. Another suspect is the contacts in the solenoid.
However last time I had this problem with that engine (forty years ago), it was the starter motor bushes worn allowing the armature to touch the poles. Relatively easy to fix provided the owner has not tried too hard to get it to start and burnt out the armature.
John
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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