How could I be so stupid? Turned out it was easy. On my way home form London on 29Feb somthing distratcted me and I left my laptop bag (of course plus contents) on the train. Amazingly I did not notice until late that evening. Must have been distratced by the Volleyball match we had to play.
The train company lost property system does not seem to work that well from the user's point of view. You cannot speak to a human. You only get through to a recorded message that says you must allow 10 days for the item to be found and logged before you can report it missing.
This is the internet age. I feel lost w/o my computer for a couple of hours! 
Fortunately I had a spare machine, unfortunately with much lower spec. Very fortunately I had a full backup (Ghost plus an external USB HDD). Unfortunately that backup is now a week old.
Spend most of a day getting the spare machine running with at least some of the software I need. Restore some of the files I cannot live without.
The money I spent on that backup solution has more than paid for itself. Even so I have lost a full week of emails and other work. Have to keep telling myself it could have been so much worse. Thank myself for being diligent enough to actually take backups.
It's a complete pain. I don't know if I should order another computer because I have no idea if the old one will turn up. I keep being plagued by finding I'm missing some software or some data and I don't have room for both. I feel like I am working in a straitjacket.
Update - 12Mar. This morning brings a postcard from Arriva Trains. 
"An item of lost property has been handed in and identified to my address." How spooky is that? I was (and am) convinced there was nothing in that bag that identified it as mine. One of the things I've been kicking myself about.
Of course I called the lost property number. I still cannot talk to a human. I left a message quoting the reference number. They have not called back yet. Of course it's only been four hours, but this is the internet age. I wanted that computer a week ago!
Update - 13Mar. Having left two messages on the recording machine no-one has called back. I try calling customer services today. The guy confirms that they have a Dell laptop computer and that it's pretty rare to have a laptop handed in although not at all unusual for them to go missing.
Update - 13Mar. The story has a happy ending. I drove to Newport. Handed over GBP25 as a release fee; and got the whole lot back. I'll need to spend some time merging back thing's I've updated onto the old machine but then things ought to be back to normal.