OK - contacted Lenovo support again. They are replacing the battery and the system board. They hope I'll get it back at the start of December. A long time to wait, but hopefully it will work properly ...
Lenovo replaced the battery and the system board. Power consumption now <10W with gentle computing. Computer is cool and quiet with a long battery life.
Very relieved! Thanks for help - gave me the confidence to persuade Lenovo to take the computer back!