Knowing that we will be without Broadband for a while, GF looked at getting a 4G MiFi but for PAYG it seemed prohibitively expensive. In the end he dug out his old 3G MiFi and switched it on. He wasn't expecting any response as he had been told the sim-card is deactivated after 6 months of non-use.
Clearly this is not the case.
The MiFi burst into life and said the account needed topping up (it is a PAYG account). £10 later, all seems to be working. So we should be set - as long as we don't try to do some massive downloads.