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I have a 1 inch gap between the walls and the bath. an anti mould sealant would not fill this and was wondering if anyone out there in england knew of any diy stores selling a plastic cover to fit around the bath preferably 1-1.5 inches deep to cover the sealant and gap from water

Yeah dude, pretty much all stores should carry a plastic bead that you can use an gun in with your anti mould sealant, look in the kitchen sections to. although it it a wide gap if you pack out underneath an seal over it will be fine.