Our house has a cistern in a fitted unit operated by a double push
button via 2 small tubes. One clear, one green.

No idea of the make or age. Only identification is a sticker KW30/05.

Tank fills to its usual level OK.

After I dismantle the valve it flushes properly once or twice. After
that about 95 percent of the time it is necessary to hold the button
down for as long as you want the flush to last.

Anyone got any ideas what the problem might be or able to identify the
kit in question? Photo link below

http://www.flickr.com/photos/billbaillie/3491110226/