I would assume the energy used would have to be "xx" months old, whether "xx" is Ofgem's 12 months or the speculative 6 months that Tomato have mentioned.
IIRC Smart meters hold 13 months of import data (and 3 months of export data), so it sort of makes sense that you can't be billed for energy that was consumed longer ago than what could be (easily) proven that you've used.
It will be interesting to see if Tomato recover from their current position.
From what I have read...
They are in default of paying for the electricity they have sold.
They have stopped taking on new customers.
They don't seem to be billing their existing customers for months - which obviously will severely affect their cash-flow.
And, if reports I have read on other forums are correct, they have recently migrated their existing customers to another company called Senapt Ltd., which is owned by the same directors as TE, but only have TE as a customer. One wonders (well, at least I do) whether that is some trick to get around regulation and/or protect the directors if SHTF.
Either way, existing TE customers won't get their electricity cut off, but the danger for users on here who need to charge cheaply is that they could be put on a standard variable tariff for months until they can be transferred to another supplier and their billing data successfully passed over.
Maybe that sounds too gloomy? I hope they pull through or well all end up bailing them out.
