Finally (as of this morning) able to post an update... and doesn't come with an NDA!

As you can imagine, we've had lots of correspondence with Virgin over this. All perfectly sensible stuff from them and well handled by them.

We explained the situation in detail. They considered it and responded with a pretty straight 'No'. (Repeat several times!)... But continued to be willing to hear us out on the matter with further correspondence until they satisfied themselves they had understood everything and our evidence was sufficiently compelling.

At that point we got a great letter from them. And I do mean a great letter. Nothing complicated, it read as simply as...thanks for talking to us about this, we quoted you the amount we intended to, but even so we recognise the information wasn't communicated as clearly as it could have been leading to a misunderstanding, so we're going to refund you the difference so you pay only the amount you understood it to be.

And that's that all sorted! (And cheque received.)

I have to say, I'm glad I was dealing with a firm like Virgin. When things aren't clear cut and someone needs to subjectively consider things to arrive at the right thing to do, you want a firm that means what its brand suggests in terms of how it feels about and treats its customers. And I think they've done that. So good on them.

And, thank you to everyone on here who helped with comments of any sort along the way.