Well, things have turned out well in the end:
I plugged it into the charger last night and reset it as it wouldn't shut down after removal from the sled. I left it plugged in to charge till this morning. It was showing a large battery icon again.
When I checked this morning, the large battery icon had gone, and the unit seemed to have shut down. I pressed the power button, and - wonder of wonders - it came back to life!!! Phew!
So I am really unsure of what has happened - but one thing is certain, it's time to RTFM, methinks.
Thanks for your help, guys.
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One of the few remaining Mk1 owners...
#00015