Raised a ticket with Shelly Support.
They sent me a firmware file and told me to recalibrate.
Managed to load firmware manually using pins which did bring the dimmer back to life.
But whatever I did the leds got stuck on at about 40% with or without a calibration. You couldn't dim or switch them off.
Support agreed it was faulty. Asked me to send it to them in Bulgaria. They would refund postage and send me a voucher for a new unit with free postage.
Contacted my Uk supplier who offered a replacement or refund. I took the replacement which I have installed this morning. So far it is behaving as it should.