Do a ramp test with all normal loads running then do a full rcd test with all loads disconnected this will give you the difference which is the leakage present under normal conditions been a tt system clamping the earth on the rod might not give you all the leakage as it may choose other routes as well, also to be noted is neighbouring premises with leakage can in some situations create nuisance tripping too when this starts been the case or similar unresolved reasons then the expensive option of main switch and full board or rcbos instead this gives easier diagnostics if fault is present, and consider several visits will start to cost alot it can sometimes be just as easy to do this for similar cost.
