Thanks to everbody who answered. A one day travel card it is, then.
I'd ordered train tickets online for a return trip to Newcastle from
East Croydon and had wanted to travel via Kings Cross Thameslink, as I
did the previous year, but the web site wouldn't allow it, so I'm
having to buy separate tickets for the E Croydon-Kings X journeys

I've since learnt that Kings X Thameslink station has closed, but that
I can do E. Croydon - St. Pancras instead. Now, wouldn't you have
thought that Gordon Brown would have personally informed me of this
important closure? When will that man ever learn?

Adam.