How can I replace the bleed plug for my towel rail?
The hardest thing to determine will be the screw diameter and pitch.  A visit to a plumbing supply will likely yield some typical samples to try.  Just don't force them in, it should start smoothly and tighten to a firm stop.

This link lists what must be common UK bleed valve sizes 3mm, 6mm, and 13mm, the ad lists all three and a key with delivery (to UK) for under £2.



How close is this to your unit?