Hi Ahoeben,
Thanks for your post. I do indeed see an Arduino Mega 2560 com port so, as you suggest, the bootloader is still in there. I would hope (especially as the machine is still under warranty) that UltiMaker would send me a suitable firmware tool to rewrite the UltiMaker firmware. I'm an electronics engineer and familiar with Arduino and other MCU manufacturers' tools so would prefer to restore the correct (UltiMaker) image.
For some reason Cura is still saying the printer is not connected so it clearly expects to see understandable responses from the printer firmware. As you suggest, I need the correct UltiMaker firmware file to transfer using Arduino tools on the PC ...
UltiMaker - over to you I think ...
Thanks and best regards - Paul.