Thank you very much for short and straightforward instructions Tony Agile - I managed the replacement without too much trouble, and without damage to the cistern.
Maybe 30 years is a bit old but hey it works. And where can I buy another green suite?
Thanks to all for your replies...
I have a 30 year old cistern and the ballcock is not shutting off the water supply. The rising water is now leaking onto the floor from the inlet pipe (which is below the level of the overflow) because I cannot tighten or loosen the pipe nut.
I would like to replace the whole assembly but...