I've never done this particular job, but, from an engineering viewpoint, I'd go to a company that makes and fits double glazing units and get a cartridge of High-modulus silicone. If you are lucky they will have it in a black colour, if not settle for the transparent variety. This stuff is essentially an adhesive and sets to a somewhat tougher consistency than the sort of stuff that you get from B&Q for sealing around baths. Clean the area where you are going to put the stuff with meths, cellulose thinners or ( my favourite ) "panel cleaner" ( from a place that sells car parts or car paint ). If you are doing the job yourself you will either need a cheap ( £3.99? ) applicator gun for the silicone from B&Q of to borrow one from a mate. You will need to let the stuff set for 24ish hours after you have applied it. HTH