Thank you for the detailed reply and picture, very helpful.
This all started with me tidying up the shower by replacing mouldy sealant but, of course, once you start these things you notice how untidy other areas are. Each end where the shower meets the wall looked really rough so I thought I’d neaten them up a bit and, well, once I started ....what was supposed to be a quick spruce up is turning into a bit of a project.