You will very likely be in imperial sizes…

www.daenotes.com/.../comparison-of-imperial-and-metric-conductor-sizes

Does this help?

Is a re-wire overdue?

Would it just be classed as copper conductors?

If tinned copper, yes. Be careful though, there was a fashion in about the same era for plain aluminium conductors which can look similar - and which have significantly higher resistance (usually necessitating going up a standard size or two for an equivalent capacity) and can have all sorts of problems especially at terminations.

   - Andy.

Worth checking if you have wooden back boxes as well?  Might be worth putting up a picture of the cable in question?