[traceroute66]

> I'm confused now, if I eat at a restaurant and they bring me the bill - is that not a private debt?

Its simple.

You chose to enter that restaurant, nobody forced you to go there.

That restaurant is perfectly entitled to set its own conditions that apply to its service to you (within reason, of course).

Hence that restaurant is perfectly entitled to say on its menu or signage or website or orally "sure we'll feed you, but we don't take cash, only cards".

If you don't like that condition, you are free to get up and leave before ordering. Just like you are free to get up before ordering if you don't like the menu, the prices or that noisy kid on the next-door table.

If you proceed to order and eat, then you have entered into a 'contract'.

The restaurant has offered you an alternative means of payment which you have accepted by proceeding to eat. Card payment was made a condition of contract.

[bloak]

You're probably right about card payment being a condition of the contract, but a contract has to be enforced by a court: the police, if they turned up, would shrug their shoulders and say it's a civil dispute. In the somewhat unlikely event of an English court being willing to accept the case and take it seriously, I would guess it would probably order that the diner pay the bill (which they could now do in cash because it is now a debt and I'm assuming traditional legal tender rules still apply), and that the restaurant pay the (probably much larger) court costs because, seeing as the diner had already offered to pay the bill in cash, the restaurant is clearly to blame for the matter coming to court and everyone's time getting wasted. The restaurant might also have to pay the diner's legal costs, if there were any, if the diner made a good impression of not having deliberately caused trouble.