I went for it and unscrewed the old one and the new one fit lovely. I was careful not to overtighten it OH came home and screwed it down further after lots of tutting about it. He has only taken an interest since I started the job myself having given up on waiting for him to start it
I siliconed round the shower tray but will wait til morning before adding water. Wish me luck
Thank you for the tips and moral support. I found one of those plug strainers under the sink when I was searching for an old toothbrush