In the open, or in some kind of outbuilding? What is the 'flooring' adjacent to the bath? Form later comments it does sound as it it's in the open.
Presumably you mean 35 V open-circuit voltage (meter on volts) and 0.12 mA short-circuit current (meter on mA). The latter reading is pretty meaningless with the short meter probes just stuck into the water and ground. Repeat the test between the metalwork of the bath (presumably earthed to the exported house earth) and a more substantial electrode in the ground - a 2 ft. offcut of 15 mm copper water pipe is handy for this.
And the house earthing system is what? TN-S, TN-C-S (PME) or TT? 35 V is quite a high difference between a supplier's mains earth and the local ground these days, with their multiply-earthed networks - but it clearly can occur. What's more to the point is that it might only take one fault in the network to turn that 35 V into 230 V and that would make getting in and out of the bath rather dangerous...
In fact the whole installation sounds quite dangerous. An exported mains earth (probably TN) into an outdoor situation and no attempt to make an equipotential zone around the tub. And the two 30 mA RCDs in cascade, no discrimination between them, sends out a message about the lack of competence of the installer.
To use an exported TN earth in this situation the very least I would have suggested is that there must be a bonded metal grid under the tub and immediate surroundings, and the bonding should meet the requirements for a main bonding conductor (min. 6mm^2 for TN-S or 10 for PME). Bits of 2.5 T&E (with 1.5 mm^2 earths) just aren't up to the job here.
The practical fix is to convert this to a local TT system. Isolate and insulate the exported earth at the outdoor end of the cable and provide a local earth system to which the tub and all its ancillaries should be earthed, via a local main earth terminal. The local earth system should consist of an adequate rod electrode (probably 8 ft.) and the aforesaid mat or grid under the tub. If the latter is impracticable then I'd advise putting in four earth rods to form an exscribed square enclosing the tub site and its immediate surroundings. These should be connected together and to the local main earth terminal using 16 mm^2 copper conductors. I presume the water supply is via plastic pipe, but any local copper plumbing that is in contact with the ground should be main-bonded to the local main earth terminal in 10 or 16 mm^2.