We've had most of the walls and some ceilings skimmed a few months
back and a few very fine cracks have appeared, not too many and they
don't appear to be getting any bigger.

Can anybody recommend a suitable filler for such cracks?

I've been using Tetrion for filling other cracks in cornicing, where I
had first slightly enlarged the cracks, and it seems very good but
wasn't sure if it was appropriate for very fine cracks.

Any suggestions?