David and Edward, thank you both.
We spent 5 hours on this on Tuesday and (surprisingly) never came across TouchOSC - thank you for that one it sounds promising with good documentation (we are an amateur theatre but we do have a realistic budget for tech). I looked at MidiMonster and a couple of others and I apologies for not putting those I have tried / looked at in my original post.
One thing that I've come across is OSC via Telnet. SFX can send data packets (assuming text messages at this point) via TCP/IP. The S48 seems to be able use TCP (SLIP) for OSC so that's something for us to explore.
It doesn't help that searching for "open sound control" largely returns items about turning on Windows sound!
Miles