.... Alas, just some workshop equipment, welder, lathe, mill... etc. 60A due to high in-rush currents of some potential future things, as well as the possibility of putting in a 3ph inverter. Plan on having at least 1 32A socket, couple of 16A and a whole crud load of 13A and fused outlets for equipment.
Have any of the electricians you consulted discussed the adequacy (or otherwise) of your house's supply to service the anticipated loads (as well as the existing ones)? There is probably also, strictly speaking, a requirement to get your DNO's permission to install equipment such as you mention.
As for the 'earth issue', as you have discovered, opinions vary, which is a sure sign that there is no one 'correct' answer, and it's all about 'swings and roundabouts' judgements - particularly in relation to which of various (all very low probability) fault scenarios one considers to be the more likely. Just a couple of thoughts ....
One consideration is that the high inrush currents of some of your equipment may well require use to use Type C (or maybe even Type D) RCBOs, and if you were utilising the exported TN-C-S earth that could cape an impact on the required size of the CPS (core, armour or both) in order to get the required disconnection times (i.e. adequately low Earth Fault Loop Impedance) in the event of L-E faults. If the outbuilding were isolated from the house's TNCS earth and reliant on it's local earth electrode (i.e. TTd), then there would be no expectation of achieving the required disconnection times (in response to L-E faults) by means of the over-current function of the RCBO, it being accepted that protection against such faults would then be reliant on the residual current function of the RCBO.
In terms of swings and roundabouts, I am rather surprised that bernard has not mentioned one issue which makes some people disinclined to export TN-C-S earths. As you will be aware, certain (rare) supply-side faults can lead to a TN-C-S 'earth' (actually the incoming neutral) rising to a dangerous potential above true earth potential. If one exports that earth to an outbuilding, one can ensure that the inside of that building is all equipotential (particularly easy when there are no extraneous-conductive parts), so that no dangerous pds shoudl exist within the building, regardless of the actual potential of the touchable exposed-conductive parts. However, if it is possible for someone to touch one of those exposed conductive parts (e.g. a metalclad light switch just inside the door) whilst standing on ground outside of the building (hence outside of the equipotential zone), then there is a theoretical possibility of a dangerous of existing between one's hands and feet - and a problem which would/could not be cleared by any protective device. However, we're talking a bout a very rare aupply fault scenario, so one has to decide whether it is worth worrying about.
BAS's initial suggestion of a 'belt and braces' approach of using the exported TN-C-S 'earth' (via core, armour or both) AND a local TT electrode will to some extent reduce that risk, since it will shift the potential of the ground in the vicinity of the earth rod to closer to the (elevated) potential of the TN-C-S 'earth' under supply fault conditions.
As an aside, even if 10mm² SWA (however many cores) could be shown to be theoretically adequate, I would personally be inclined to go a fair bit larger than that (perhaps 25mm²), not to least to avoid the need to dig up the garden again should the future bring different requirements!
Just a few thoughts!
Kind Regards, John