Bath Waste?

My bath is leaking due to the nut failing on the bolt that connects the upper plughole cover to the lower piece of black piping underneath the bath (the housing in the black pipe, which holds the nut has also failed), on buying a replacement I find that it is too long and will not fit within the space available, It would seem that the original plumbers used a shorter version of this presumably for restricted space, unfortunatly now where seems to stock this version. Where did they get this part from ?