Cistern keeps flushing rapidly?

hi I have a concealed toilet cistern (white square box) with a dual flush push button and I can get access to the float.

The problem is that after a big flush the water keeps running at significant speed - so much that it is emptying as quickly as the mains can supply.

I suspect it being a mechanical error more than a leak as a little flush does not cause problems and the toilet water is not otherewise running.