Thanks for all the replies. I'll try the WD40, but as I said it's been like this from new. I've got average hand strength but shortish fingers, and I'm pulling the lever with my finger tips. I wonder if an adjustable lever would help?
When on the bike have you checked that the line down the top of your arm to the lever is straight? incorrectly angled levers can lead to rsi type fatigue and pain making lever action harder than it should be.