My cistern Macdee compact duo also was letting water into toilet.
Turned off water, flushed toilet.
Unscrewed Float valve assembly at inlet.
Took apart each bit of valve and cleaned any gunge, remembering the order and orientation of parts.
Unhooked the little lever-valve that the float pushes up and cleaned.
Re-assembled all parts keeping them rotated in same orientation as before.
Refitted assembly, turned on water, problem has not recurred.
Andy
July 2011
get a plumber if you can find one.They usually leave you with more damage and problems than before they came.
Buy a new cistern and put a pin in you local guide for local plumbers
Meltdown
April 2009
I had the same problem. The only way that I could fix it was by replacing the inlet valve with a compatible part from www.bes.co.uk part number 15887 Fluidmaster side entry valve £7.25 plus delivery and vat.
It took me about 20 minutes to change over and the result was a silent non drip cystern.
Good luck.