It pays to remember these things! If you forget, the latest band plans are here.
After a little while (spent facepalming at my ineptitude) I managed to coax Alex down to 5.357800MHz by sending "WORK SPLIT?" and we made the contact.
Whilst sending "WORK SPLIT" via JT-65 I had more than enough to time to visit wikipedia and discover that sending "QSW?" would have probably been more conventional:
QSW? - Will you send on this frequency?
My KXPA100 has arrived and is working nicely. I needed a few more watts of grunt to compensate for my somewhat short transmitting antenna. Happy days!
I've only just noticed that it took nearly half an hour to make that QSO! Still, I find it best not to rush things.
Update: Philip, G0ISW, has a nice article about doing this the right way.