Thanks. I note that they have drivers for the 940C series but not the 840C, but I'll download the dmg to which you pointed and try to get it to work over the next few days.

I shall report back the results in due course.

Thanks for the advice.

iMac (Retina 5K, 27-inch, 2017); 3.8 GHz Intel Core i5; 2TB HD; 64 GB 2400 MHz DDR4 RAM; Radeon Pro 580 8 GB. iPad Pro 10.5" Model MPME2B/A with 512GB capacity; iPhone SE 2020 256GB