It appears that the electrician's position is that the only way to make the hot tub installation compliant is to make the complete house installation TT which seems a drastic step.

Not the only way, but certainly one possibility. Depending on the setup it might not be that difficult - e.g. shove an S-type RCD before the main CU and re-route the earthing conductor from the DNO's earth terminal to a local electrode. On the other hand if you have extraneous-conductive-parts shared with other installations (e.g. metal gas or water supply pipes) (or place your TT electrode near such items) then PME fault voltages can still get into your installation and TTing it becomes a bit of a waste of effort.

What I have taken from this forum so far is that as long as there is appropriate separation, 2 metres minimum, between equipment, sockets, lights, heat pump that are PME from the TT hot tub that is compliant with regulations. The other sugestion that came up was to put a physical barrier between the TT hot tub and the PME heat pump which is also practical. Have I got this wrong? Is the only way to be compliant to TT the whole house or is physical / distance separation compliant with a resonable / practical interpretation of the regs?

I don't think the regs specifically say 2m (rather just not 'simultaneously accessible') - some might suggest 2.5m (1.25m + 1.25m for 'out of reach') - but that's between exposed-conductive-parts (e.g. metal cased electrical equipment) - a socket is more "interesting" since the measurement would have to be to something plugged into the socket rather than the socket itself - e.g. a class I electric patio heater - so a bigger gap might be worth considering (or moving the socket). While we usually do measure around barriers, common sense would suggest that would have to be reasonably permanent to ensure they remained in position for the life of the installation.

The other options the regs give is to use PME but add an additional electrode of 20Ω or less (as distinct from a TT electrode) - personally I'm dubious that a simple (rod) electrode would give much practical benefit - although in the form of a buried grid under the hot tub and immediately surrounding area could work. Whether that is acceptable under the manufacturer's instructions is another matter though.

   - Andy.

No gas to the house and all water pipes on the outside of the house are plastic as property only built 2 years ago.

So we have finally come to an acceptable solution for the electrician that should satisfy the hot tub supplier. The hot tub and heat pump will be TT with the PME earth ending at the rotary isolator (hot tub) and fused connection unit (heat pump) both of which are more than 2 metres from the hot tub/heat pump. Apart from these the closest PME earth outlets will be a wall light and outside power socket both of which are 2.7 metres away from the hot tub. To reduce the impedance on the TT earthing there is a going to be an additional 1200mm earth rod put in the ground making 2 in total. 

Thanks for the input final decision is to TT hot tub and heat pump.

The hot tub and heat pump will be TT with the PME earth ending at the rotary isolator (hot tub) and fused connection unit (heat pump)

What of the heat pump control wiring? Presumably that connects the heat pump (outside) with things within the house? If that wiring carries a c.p.c. then either it'll re-introduce PME potentials into the TT zone or it'll risk introducing TT potentials into the PME zone.

  - Andy.

My understanding is that heat pump control wiring is only between it and the hot tub. The only connection to house is the feed from the consumer unit protected by through the rotary switch and the fused switched connection unit. Both are on separate MCB's protected by RCD. The earth breaks at the switches and there are no other connections back to the house. The hot tub installers have confirmed they will not touch any house wiring only connect the hot tub and heat pump to the weatherproof connection boxes on each cable from the switches.

John Ford said:

My understanding is that heat pump control wiring is only between it and the hot tub.

What is the supply arrangement and system voltage for the controls?

If not SELV, may have some issues going between the two earthing arrangements?

My understanding is that heat pump control wiring is only between it and the hot tub.

Ah, so the heat pump is dedicated for the hot tub? Not part of the general building heating system?

  - Andy.

Yes sorry should have been clear the heat pump is only for the hot tub.