Pretty much as Neil said, Its usually a pawl which is sticking or a spring thats gunked up. If i remember this weekend I will take some pics, I am bringing some shifters back to life.
I purchased some de-greaser from my local motor discount shop (£1.99) and give it a liberal spray, the gunk breaks down and can be washed out with some warm soapy water, thoroughly dry, light lube and grease. Exercise the levers through the full gear range and then refit cover, good idea to fit new cable whilst you have with the cover off, can make the job easier. Then just put the shifters to one side ready to re-fit as required.
"Would anyone else rather be doing something other than marching up and down the square????"