I should say the chip inside is the moschip AX99100 which seems to be up to the job. It has SPI and most features I understand but do not know if linuxcnc uses...
Thanks again and I will no doubt be back to bother you all when it arrives and I can't get any joy! Happy new year to all.
AX99100 in SPI mode sounds interesting if used together with Remora or RIO. I think there was a port of the component from bcm2835 specific library to a more generic spidev, which is implemented in this driver:
github.com/daveriesz/ax99100