So yours was 30 metres and £600, the OP's is 25 metres and £450!
Your talking rubbish, no one can say its expensive, no one can say its cheap.
I've just priced up a similar scenario which worked out as just under £2000.
You cannot export a TN-C-S supply.
You will need to isolate the armouring at the shed end, and fit a plastic CU with RCD main switch, and earth electrode.
I think you need an electrician :shock:
You will need more like 16/25mm cable, which at your distance will be £400-600 alone.
Then you'd need to consider exporting PME, and RCD requirements.
No doubt when people start getting electrocuted he'll be looking at someone to blame, and I'm sure as eggs are eggs he will blame someone other than himself. to55er