Sarah Brown wrote:
Will the ticket offices at Cambridge sell that one?

They should do: all stations should sell all tickets available on the
national network. (Excluding 'experimental' fares like Megatrain). I'm not
quite sure of where to find the rules stating that to wave at them though.

Failing that, I guess I could always either drive to Ely (despite living
within short walking distance of Cambridge station), or buy a single to Ely,
and then a travelcard once at Ely.

You could also buy your tickets on the internet and collect them from the
machines at Cambridge once that's enabled (though I note they currently have
notices saying pre-booked tickets should be collected from the ticket
office, which surprised me since Qjump etc don't list Cambridge as a
collection point so I thought collection wasn't officially provided at
Cambridge). Or buy them in advance from the advance purchase window.

Theo