Thank you for your reply NFH.
Another question: what types of voucher can I use to top up an EE sim? I mean, if I got a T-Mobile voucher, could I use that to top-up an EE PAYG sim?
Alternatively... could I use a Neteller prepaid card to top-up a UK number? Otherwise I'll buy one or two spare vouchers next time (really soon) I get in the UK