morddwyd - many thanks. At the moment I cannot find a Google Checkout screen. I have Google Wallet with a debit card set up for payment which I thought should be enough.

I am beginning to wonder if it is because I have 2 g-mail accounts set up on my phone, one is ordinary gmail, the other is @sky.co which operates through gmail. I want to delete the latter and set it up as pop e-mail instead because that may be causing confusion, but the phone is not letting me do that. I may have to resort to a factory reset and start again. That's not a major issue because it is only a couple of weeks old and so there is not much to restore manually.

morddwyd, thanks again for your input.

I carried out a reset and now have 1 gmail account and a POP email. That seems to have done the trick and I can now purchase away to my hearts content.