My upright has developed some creaky dampers over the past couple of months, possibly to do with reduced humidity levels (from mid 60%RH to mid 40%RH) or "settling in". Can anyone suggest a fix?

There are various possibilities. The first thing, I think, is to carefully identify where the creak is coming from.

Is it definitely only the odd damper here and there, when depressing individual keys? Or do you hear a creak when you depress the sustaining pedal?

If you hear creaking when you depress the sustaining pedal, try taking the pedal and rod mechanism out of the equation by getting your hand in and lifting the damper list rail lever with your finger. (If there is not room to do that, you may have to unscrew the action and tilt it forwards, or even take it out of the piano).

If there is creaking when you use the pedal, but not when you lift the lever with your finger, then the problem is somewhere in the pedal or the interface between the surface of the vertical damper rod and the underside of the lever on the action. Listen and try and identify.

If the creaking happens when you lift the damper lever on the action with your finger, there may be a combined noise coming from friction between the metal lift rod that runs along the action rail, and the action cloth on the backs of the damper levers.

If the creaking is very specific to just certain notes, then it may be the spoon against the action cloth on the damper tail (check for wear - it could be worn through to the wood), or it may be the damper spring in the damper groove.

For creaks originating in the damper pedal bracket, use a suitable oil (about the only thing in a piano that you can use oil to lubricate). For all the other places, you might try judicious use of Teflon powder.

I have known of situations where there was a general creak from the combined friction of damper lever cloth against the metal damper lift rod, and the noise didn't go away with careful application of Teflon powder to the interface between damper lift rod and damper levers. In those situations, taking the metal damper rod out, polishing it with metal polish, then wiping it with a clean cloth dampened with Protek CLP cured the problem.

Emphatically, do not try spraying anything in a general way inside the piano!