Thanks for that; I have confirmed that it is the calibraiton. I loaded the blink sketch and the LED flashes very slowly.
I have tried calibrating using https://github.com/MCUdude/MicroCore/blob/master/avr/libraries/Serial_exampes/examples/OscillatorCalibration/OscillatorCalibration.ino. As instructed I send x's but all I get back is ???? so I needed to do some more work here, or use one of your links.
Will try again later.
Alas I will need to wait until our lockdown lifts... I need to try a different multimeter.