It looks like the type of reducer fitted to 100mm Vitrified Clay Drainage, (Hepworth HepSleve or SuperSleve), so may be worth bearing that in mind when looking for a replacement. If you can check the drains outside that may confirm the material used. Just be careful removing/refitting cap is it's a cut end of pipe sticking up, the edges can be razor sharp if not chamfered properly when installed.
