Cheers guys.
I have had some success now. I pulled off the old push fit, and one of the new traps I bought does fit the outlet. Unfortunately that one doesn't fit the sink outlet. But I am hoping it will simply be a case of swapping it for one with a larger inlet section.
Thanks for your help.
I am trying to replace the waste trap beneath my kitchen sink. The old, broken one is labelled :
Marley Extrusions 11/2 / 40 PP BS3943
I cannot find this brand anywhere.
I showed the old trap to the local plumbing merchants and they sold me a McAlpine trap, saying it should definitely fit...