No luck with JetCombo. Eventually I had to call Aeroflot Airlines in Moscow to find out what was going on. They confirmed that there was a problem with the website. Fortunately they were able to give me a local number (for Tokyo) (03) 55328781. Japanese and Russian spoken only, however (thankfully one of us spoke Russian).

Ticket successfully booked

A Google search for the phone number of the Tokyo Office proved fruitless (all numbers out of date), but with the number known, I was able to come up with this link (for future reference for adventurous travellers)

http://www.aeroflot.aero/cms/en/about/toy

Cheers.
Ian.