There is a chemical product you can get in Focus. it is in a tube in the fillers section and it is a silicone remover. It stinks like hell and wear gloves. Sorry cannot remember the make. Homebase sell it too. :)
You can buy silicone remover from screwfix. I've used it a few times, it works really well but may need a second application if the sealant to be removed is quite thick. I would avoid using too sharp a knife to remove sealant as you can nick the edge of a plastic bath/shower of even cut into the aluminium. See the removing sealant project.