So I went through the remove watch from system/reboot/reinstall ritual everyone will be familiar with and things seem OK. I even managed to transfer a swimming mime so it worked on dry land. I'll have to see how it goes next session, I've just checked and the watch is around 18 months old so out of guarantee.
It is a pity that the data was lost as I thought I'd done some pretty fast lengths and sustained the pace through the session. But as I said last time, it's the effort that counts, not the numbers. For this session that's what's written on my training plan by the coach too.