m_c,
Thank you so much for that, it's cleared up a lot including exactly what Clive S meant in this comment from a thread I started last year...

"I am not sure why you sure why knocking linuxcnc saying that it "especially one requiring an antiquated parallel port" as that is not true. Stick any Mesa card in it and you will have pulse timing as fast and accurate as you will need. If you want Ethernet connected card then use a 7i76e card."

I've had a look around for information on using LinuxCNC with a MESA card and came across a YouTuber called Marco Reps in Germany. Slightly eccentric (I liked him immediately) he has an interesting video on exactly that. It drags on a bit (14 mins) and the first minute or so are not relevant but he goes on to describe some of the tools in LinuxCNC which I had no clue about, some of which are only used when tuning servo drives. He recommends the MESA 6i24 card

Kit

PS I should probably appologise to Voicecoil for starting what has amounted to a complete hijack of his original thread.