Aw bollocks! I found a Thunderbolt 2 to FireWire adapter very very cheap, so got the matching cable and the Thunderbolt 3 (USB-C) to Thunderbolt 2 cable. Everything connected, the Mac recognises the FireWire interface, so far so good.
Sadly, the old JVC Mini DV Camera we used back in the day doesn't function properly anymore. I knew the FireWire interface was probably botched as I couldn't get it to work under Windows. But on top of that the thing is blind.