A_camera, Doddy, thanks for replies.
I think the existing system will have the opto isolation built in,I'll try to attach Ist page of manual. (I got it in 2012 and the guys website doesn't seem to exist any more)
Click image for larger version.  Name: page1.JPG  Views: 729  Size: 89.5 KB  ID: 29708

If I don't need a new BoB I guess the AXBB if probably overkill for my setup and I'd look at the UC400. In the UK I can only see them for sale at CNC4YOU and Stoneycnc in Ireland. Are there any other outlets you guys are aware of?

Cheers
Ian