Yoou can imaging the humooour oof text that has the ooooletter "o" liberally spirinkoled everywhere. I had to just press a key nearby or lightly brush the actual 'o' key and I had unwanted letters. Actually pressing the key would likely result in ore vowels that were required.
An Ebay search revealed that you can buy replacements, so I took to it with a screwdriver to see if I could improve things.
I released the keyboard by releasing the three very small catches above it - shown below:
Oddly, instead of a small screwdriver I chose a butter knife to release them - just press them in, starting at one side, then the middle, then the other one. No need to pull the cable out of the back unless you are replacing the whole thing.
I then flicked the offending key off by pushing the plastic catches out from below. I held the underside of the key up with the tiny screwdriver I should have had all the time and blew underneath it - I have read this can be caused by dust between the key and the membrane.
Then I took off the plastic bit from the back of the key and attached it (with, I admit, some difficulty - I had to push the metal catch up a bit first). With the plastic bit attached okay, the key just clipped in place.
I am pleased to say that my "o" key is no longer over-enthusiastic!