Thanks to all for your responses.
I don't know how long the car had been sat idle before I bought it. I did about 800 miles, mostly long trips, after purchase, but it was then garaged for 15 days while I was away on holiday.
I don't think there's any problem because, within about 30 minutes after taking it out, the auto stop/start resumed its normal function and there have been no further warnings.
However, I'll probably be leaving it in the garage for several days at a time over the forthcoming winter, so I've ordered a CTEK MXS 5 from Amazon - can't hurt to do a weekly recharge.