although this has been like that for years you have been messing about inside the cistern today and could have made it worse, you need to change the split nut ASAP it could sheer and flood the place. if the ballvalve overflow pipe and syphon are all properly positioned they wont hit each other so either the ballcock or overflow pipe is at the wrong angle, drain the cistern, remove the overflow pipe and change the nut (3/4" required) refit overflow and see if it can be positioned away from the ball, some are angled so position it away from the ball, if it is a straight pipe you might need to move the ballvalve round a bit, while the cistern is empty loosen the nut securing the ballvalve to the cistern and turn the ballvalve from the inside so that the ball lifts and drops freely, tighten everything up, dry it off, turn on the water check everything THEN go and have your glass of wine, if you cant get a new nut from anywhere tonight i would seriuosly consider draining the cistern, (just in case)