Sorry if this sounds obvious, and if it's already been tried but have you tried to register with another phone? (i.e. another IMEI). Sometimes genuine IMEI's flag up on international databases as being 'questionable' for all sorts of bizarre reasons. (too long winded to explain here).
Thanks for the suggestion petkow: I had only tried the IMEI of the mobile in which it was going to be used.
Anyway, trying from a different internet access point I got a different result.
Sorry this voucher cannot be validated online: please call customer services on 0845 880 1234.
I have not had the chance to do so as yet, but at least it is some progress to get a number on which one can call !