loo flushes for ages !!!

[Crayola]

hi there

when i flush my loo, the cistern thingy keeps on taking in water for about 10 sometimes upto 20 mins.
with the cistern thingy being full - the water just comes out into the loo.
its doing my nut in - especially at 3am !!!

my loo is one of those that u flush by pressing a button on the top

any ideas?
is it a simple thing or do a need a professional ?

 

[Plumber Will]

THe float is sticking, it may be catching on the side of the cistern, or it may need adusting, the float will either screw up and down or be on an arm at the end of the arm there is an adusting bolt or screw.