This should be fun... Actually I think the former is actually the pattern, pattern for the shape I want to form. The mould is the thing into which you pour the, for want of a better word, stuff you want to form into a given shape. Dictionary definition is: 'a hollow container used to give shape to molten or hot liquid material when it cools and hardens.' So, @jack would say never start a sentence with so, now the mould in this case is the expanded polystyrene slab former supplemented by the shower tray former, which together FORM the mould. Ergo, ipso facto, inter alia and among other things and to be perfectly pedantic - until @Nickfromwales or some other master of both English and the language of the technology of casting concrete tells us otherwise, the former is just / only part of the mould and as such is FORMING part of said mould. Let the fun commence, I will look for a pin the head of which I can throw some shapes upon