The forks seem really stiff and even after months of riding they are getting no softer.

Am I just too light to get reasonable compression? 70kg.

Is there something I can do – say new lighter coil spring or is the oil damping likely to come in different weights and could be changed for something thinner?

Get a lighter spring. Most people will need to change springs on coil forks to get the right one.

ok thanks. Called tftuned softer coils in post.