I'm trying to find some kind of union with a M12 male fitting one end and a tube to accept a hose (size not important) at the other.

It's an overflow outlet so almost any material would do the job.

Any ideas?

Ta

Bore an 8mm (ish) hole through a bolt, then cut off the head?

Paul G

Acorn Fluid Power in Cwmbran.

You could probably use a compressed air fitting.