I am finding recently that spending even relatively short periods out of the saddle with my knees bent, e.g. when descending on rough trail, my calfs start to ache pretty quickly.

Why is this and what can I do (e.g. what exercise)to stop it from happening?

Watching with interest.
I too find my calfs and thighs hurt more on descents than climbs.

Move your cleats further back. Drop your heals.

what he said ^^^

I’m a well known mincer on descents, so I’d say it’s tensing up…holding the bike with you lower leg.

I find it hurts more with my heels dropped and I haven’t noticed the pain when riding my other bike with flats as my feet tend to be placed further forwards with the pedal axle under the bridge of my foot.

my feet tend to be placed further forwards with the pedal axle under the bridge of my foot

So move your cleats back, as suggested…

Doh!, I read it as move feet further back!

Yes, of course moving cleats back will move my feet further forwards.

I’ll give that a go, thanks for suggestion.