Maybe dig a hole 2 or 3 feet deep and whack a 2.3 meter one in as a test rod and see what readings it gives you. It might be an hour and the cost of the rod well spent. If you get a half decent impedance reading just attach another one or two rods to make it deeper and get the necessary readings. A TT might save you a lot of time and money especially with the cable being 240mm².
Thinking out loud, I'm wondering why the original installer didn't parallel up two smaller csa cables. I'm also wondering what the earth impedance would be ideally for a 240mm² SWA cable when using the armour. I don't have tables to hand but it might be worth looking it up, over 100m the reading your getting might not be far off the mark in which case no amount of remaking the joints is going to give the result you need.