Thanks for all the replies. I decided to go with The Novice's advice first as that seemed easy. I googled siamp washer, saw a link to screwfix and a YouTube video showing how to fit them. Bought the washer, watched the video, followed the instructions and now the cistern fills in about 40...
The toilet in my bathroom fills very slowly and I don't know what to do to make it fill quicker. I'm posting a picture so hopefully someone can tell me what I need to do.