Right, I've had chance to look at this, and pleased to say, fixed it, up to now thankfully. It did actually come off without removing the cistern, just gave it a little turn as seco services said. And, at the bottom was the large white washer (flapper?) again, as seco services said, thank you very much. The whole of the flusher comes out so I cleaned it all, as well as the button which I took apart too. Put it all back, and it was just the same. So, I took off the white washer/flapper and turned it over, and put it back on. That instantly fixed it, and it's been ok since. Though there was no sign of any damage or wear, it was obviously 'set' or had become 'used' to it's postion, and flipping it over has given it new life. Not sure if this is a permanent fix or not, but it's fixed at the moment. And now I know I can take the whole thing out it's easy enough for me to replace in the future. Thanks for all the help!