Thanks to all who've offered advice. @HilaryBriss > "You usually just get sent a code via text message that you then enter into the relevant box on the website. Are you saying that you are not receiving texts?"
I use a laptop to buy online; can't use my Windows phone to do that so never have. I have had codes in text messages lately, but didn't realise there was a box on the website to enter it into - will look out for that.
@BarbaraofSeville > I used online banking for several years, so certainly nothing against that. For some reason when I tried to access it a few years it wouldn't allow me in, despite nothing having changed with my details. Got in touch with the bank and they sent me an email supposed to contain a link to get back in, which would expire within 24 hours. The email did not contain a link. I tried several times to get online banking up and running again and in the end gave up in despair. So; I was originally registered for online banking. My mobile network is Tesco, but it's pay as you go. I don't use my phone nearly enough to have a monthly contract. I never get through my free credit when I top up before it runs out.
I do now have an android phone and need to inform everywhere of my new number. The problem is, again, I don't seem to be able to change my contact details online with my bank. I need to give the original number they hold to do that (and I've been with the bank so long I don't even know which one that is), but I can't, because I don't have access to online banking to alter my details. To reiterate, I did use online banking for this same account, but then I no longer could.
Obviously I'll be ringing the customer services line to update; but I've not done that yet because a week ago I was on the phone for one-and-a-half hours to a call centre in order to verify whether an email from them about whether an email I'd received, which looked totally authentic, could have been a scam. Turned out it more than likely was. So I'm just putting off that lovely experience.
For those who think it's a case of 'just get a new phone'. If only life was that simple.