You could load a photo as rpm suggested, but It’s probably a 3 or 3 and 1/2” connection, a multikwik or McAlpine connector should dirt your problem. MKS3 or MKS10 for the multikwik’s. You could measure the size of it to confirm.
Manufactured from EVA ensuring it remains flexible past the point of installation - ideal for tolerating movement and WC load bearing pressures.
www.hunterplastics.co.uk