I've just discovered a leak in the connection between the isolation valve and plastic thread on the inlet to my toilet. I've disconnected enough to see that the fibre washer in the connection has split, so I'm assuming that was the problem. Is it normal for a fibre washer to split like this? I also noticed having removed the washer that it was very spongy and brittle. There was slight movement in the cistern (a little wobbly) after I'd replaced the flush some time ago - the new doughnut wasn't quite the same size as the original and I couldn't screw the cistern to the wall as well. Maybe this contributed to the leak. I will probably just replace the fibre washer with another and try and secure the cistern better. Apart from that any advice how I can stop it happening again? Thanks.