So in summary you are suggesting that would be sensible to have a TT earth on the hot tub?

From what you have said, if you can step out of it onto bare earth, yes. But as a separate thing, when you stand next to and touch the heat pump, what are your feet on ? If it is bare earth, then easier mitigations than rewiring it may be some gravel or concrete paving slab - one discourages bare feet and drains water away quickly, the other acts as a current limiting resistor in series with the body. Neither are in the big book of best practice, but are ways of turning a small risk of a bad shock into a small risk of just an annoying one.

There must be no metal connection between the house earth and the TT earth, and where they come close - probably in a wall mounted box near the  hot tub then one or other should be insulated as potentially live part.
Common to use an armoured cable, where the armour connects to the supply earth, and operates a fuse or breaker if the cable is found by pick axe or drill, but at the load end the armour is 'gapped' and a plastic box and gland are used. From that point on the earth wiring is relying on the grounded electrode(s).
There must be an RCD before or at the transition to the TT island, and that RCD must be tested regularly.

Page 22 of this PDF has some info about electrodes and resistances, and the voltages near electrodes during faults that may be of interest.

Mike

Appreciate all your advice Mike. The plan was to place the air source heat pump on paving slabs given what you have said I will probably extend the slab area around the pump so it is not reasonable for anybody to touch it while on grass/bare ground.

The electrican has used SWA glands and all wires within the rotary switch are insulated. The rotary switch is protected by a 30ma RCD in the consumer unit with PME, the break in the earth for the rod is inside the plastic rotary switch housing. At the request of the hot tub supplier the heat pump is fed by a separate circuit from the consumer unit to a fused switch using the continous house earth, house PME, to pump. Should have said at the outset that the SWA cables x 2 (hot tub and heat pump) from consumer unit, rotary switch, fused switch connection unit and earth rod are all installed but not yet covered or final. So this chat is a little late but not too late that things cannot be modified to meet regs and be as safe as it can be.

Appreciate all your advice Mike. The plan was to place the air source heat pump on paving slabs given what you have said I will probably extend the slab area around the pump so it is not reasonable for anybody to touch it while on grass/bare ground.

The electrican has used SWA glands and all wires within the rotary switch are insulated. The rotary switch is protected by a 30ma RCD in the consumer unit with PME, the break in the earth for the rod is inside the plastic rotary switch housing. At the request of the hot tub supplier the heat pump is fed by a separate circuit from the consumer unit to a fused switch using the continous house earth, house PME, to pump. Should have said at the outset that the SWA cables x 2 (hot tub and heat pump) from consumer unit, rotary switch, fused switch connection unit and earth rod are all installed but not yet covered or final. So this chat is a little late but not too late that things cannot be modified to meet regs and be as safe as it can be.

Sounds like you have it under control, but do let us know how it goes. If the SWA armour is PME earth then the gland nearest the break ideally needs to be plastic or over sleeved, but in practice folk do not go around touching cable glands, and the risk compares to the average outside tap, also often PME earthed, and sometimes above wet ground.

Which raises another thought - how does water get to the hot tub - somewhere a plastic pipe may be needed to gap you if the house plumbing is all metal.

Mike.

Reasons NOT to install a hot tub.

1.Earthing problems.

2. High cost of heating unless you use solar panels and no electricity.

3. Dangerous water with chemicals added and germs.

4. Unwelcome friends and neighbours want to come and use it free and don't even bring a bottle of something or snacks.

5. Expensive to buy.

6. Bad for the health in the winter.

7. Removing broken wine glass shards at bottom of hot tub after party is no fun.

8. Vomit floats on surface of water. Most off-putting.

9. Weight gain.

10. High maintenance.......

Tips For Having a Great Hot Tub Experience - Bing video

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A sobering tale written in foreign just to make it more dramatic...

Short story: A hot tub disaster | HeraldScotland

Z