Marquee with a TT, Easy?

Gents, I'm going to be putting temporary electrics into a Marquee in a garden centre. The marquee is being plonked next to the garden centre building and will be sitting on brick block work. To use a rod will mean running a cable to it about 30metres across the block work to an area where the rod can be sunk into the ground. However, the public have access to this whole area which makes running the cable even in cable mat protection problematic.

The building next to the marquee has metal framework sunk into concrete foundations, My initial thoughts is to use this as the "rod", BUT this metalwork is bonded to the PME system in the building.

So my question is, can the metal framework be used?

7671 permits framework to be used as an earth electrode, and it could be a very good rod too with nice low readings

you might want to look at BS7909 which is for temporary power for entertainment, its probably out of scope for your marquee depending what you are doing with it, however it does have similarities which may be helpful. also worth noting this from one of its annexes

TN-C-S source, supplementary earth electrode for the temporary system
If practicable, where a temporary distribution supplied from a TN-C-S
source is used outdoors, then the correct deployment of an additional
earth electrode in the vicinity of its use will be of benefit as it will assist in
reducing the effects of a PEN failure.

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