Ian, I apologize - I had made an (I think) invalid assumption - sounds like your setup has the BoB and Drivers installled onto the single board (again, an image would help but it certainly sounds that way - and I am familiar in passing with that sort of board). But, yeah, you should be able to plug a UC300/UC400 into that without any real surprises. Again, you'd need the ribbon cable, but that's all doable.

I bought my UC300eth from CNCDrive directly with no problems (pre brexit/corvid), and an AXBB from another member of this forum, but that's for themselves to offer if the opportunity remains.

Unspoken, but going for those controllers opens up the option of replacing Mach3 with UCCNC - most people that have tried this seem to prefer it to Mach3 - I certainly do. You can download a copy of their software - without license file it only operates in a demo mode, but enough to get a feel for it.