As a follow up on this, a friend asked his accountant to get him a Greek tax number, ΑΦΜ. Apparently he could not get a Residence Certificate unless he had an ΑΦΜ.
The accountant has told him that in order to get his ΑΦΜ he must provide an official letter from the UK authorities stating his UK Tax Identification Number, TIN, and the letter must have an Apostille Stamp. In case anybody else hits the same problem this government web page
details how to get confirmation of your National Insurance number, which acts as a UK TIN, and this government web page
details how to get an Apostille Stamp. It is vital that the letter confirming the NI number has a printed name of the person issuing the document, a signature and that the signature is the person whose name is printed, i.e. NOT pp. It helps if the person doing this already has their signature on record at the Legalisation Office.
I realise getting a bank account without an ΑΦΜ and getting an ΑΦΜ are not exactly the same thing but they are closely related.