Belboz and Daniel.
Thank you do much for your generosity in sharing the links with me. Sorry to say I had to call in a Pro to sort it out as my wife was on my case for spending too much time on it,but next time I will be ready. Once again thank you for taking to respond to my query...
Thanks Belboz. Yes the water supply can be easily shut. I must confess I have never disassemble a cistern before. You may be right about the flap valve being worn out or failed. I realise that when I push the ball down manually to increase the hydrostatic pressure, the cistern flushes alright...