Robin 4102 Earth tester

I have a Robin 4102 Earth Tester which tests the impedance of a ground earth connection, it comes with two earth spikes which are connected at 5 and 10
metres plus the earth rod under test, the meter is an analogue type.
This unit was donated to the radio club that I belong to. However, the batteries were all corroded and needed a new battery connection. I have cleaned up the battery case, some of the PCB was also corroded so I have replaced the battery connector and resoldered the dry joints etc.
The unit does a battery test OK but just sits in mid ish position when switched to measure, I have tried a decade box across it but it just sits in the same position.
Does anyone have any instructions, service manual or just the method of testing one of these earth testers.
Many Thanks
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