........'cept it's another engine seize not flat battery.
Was puzzled 'cos dying battery voltage usually results in backfire, etc and then engine slowly dies. This time, engine went a little 'fluffy' and then died.
Checked out all electrics, all good. Manually turned engine and hit lock point - turned crank back as far as possible, hit starter, brief spin stopped with loud clunk - guess it's dropped valve again.......
Will strip out top end tomorrow.