Toilet cistern won't stop running

[Mrstrumpalot]

Just as title says really. It's making a continual running water sound. I have taken the top off and stuck a toilet brush in to keep the float up.
What do I need to do please?

Replacement cistern part. Go to Homebase/Wickes etc., tell them the problem and they will sort you out.

Look to see if you have an isolation valve on the water inlet and switch off water to sister's.