I have some shelves that I want to put up. My walls are plasterboard and are 45mm thick. I can't find any fixings that work with this thickness, as all cavity anchors appear to have a limit of around 35mm. Any suggestions would be very welcome.
Plasterboard does not come in a 45mm thickness unless its backed by polystyrene as an insulation board. If this is not the case it might be that you are actually on a timber stud. The best way to get a decent fix if it is insulated board, is to cut out a section of the board to insert a timber which you plug and screw to the wall and to which you fix the shelf brackets. The timber can be painted to either match the wall or stand out as if it were part of the desired shelf construction.