Some good news today, I managed to source a Diesel tachometer from a Jetta. Had some time to fiddle and test it with the latest PCB prototype this week and although it's not a perfect fit, it works!
I may have a go at swapping over the PCB from the Jetta tachometer into the T3 tachometer I have. If it works I should have a functioning factory tachometer on my T3.