B&Q diy toilet tank not staying full after flushing?

Hi all- I bought, and installed myself, a cheapy toilet from B&Q. ('Toilet To Go')It has worked fine for about 2 years until recently. Unless you flush it fully- ie hold the button down and totally empty the tank- the tank fills up and somehow, water seeps into the bowl. The result being the inlet/float valve compensating for the loss and topping up every 2 minutes or so. Very annoying! The water is not going down the overflow. It fills up about 2mm short of that. Like I say, a full flush seems to remedy the problem but try telling the kids!
There is a plastic screw sticking out of the flush mechanism. Some adjustment called for maybe? What does that screw do? Thanks in advance bog buddies...