The pump had begun stopping and starting much more frequently and for shorter periods than usual; so I decided to add more air.
I switched off the pump (top picture) then drained off the water in the tank.
Then I connected my bicycle pump to the valve on top of the tank
...and pumped it up to about 15 psi. My bicycle pump has a useful gauge on it.
...and now it is working as well as it used to. My friend Philip, who knows a lot about pump systems, suggested that the diaphragm in the tank may be ruptured, and suggested replacing the tank under warranty. But for the moment, I'll see how long this recharging will last....