Just to be completely clear, it isn't exactly what happened before.
Previously the range dropped from about 20 miles to 0 and then after cycling the power slowly recovered back to where it should be; there was an engine warning light and error codes. Cambridge Chevrolet replaced the battery and I believe cured that problem.
Now, I have no engine warning light and no error codes (I believe Chevrolet UK think there can't be a problem without an error code, Bellingers, the Volt dealer in Oxfordshire admit they have seen a problem but can't find the cause and can't go further without Chevrolet UK agreeing to look at it). Sometimes I'm losing a chunk of range, but power cycling the car doesn't get it back. Sometimes I'm not losing the range but the car is acting as though the battery has been over drained. This looks to me as though the actual battery capacity is reduced but the system that monitors remaining capacity isn't detecting that reliably.
There could be some overlap between these two issues (perhaps the fault is with the controller), but the more I think about it the more I think it is simply that one duff battery has been replaced by another differently duff battery.