Basin waste

[DanielPatrick]

you're right it's a standard 1/1/4" basin waste

 

[DanielPatrick]

Thanks for clarifying, it is a standard 1/1/4" basin waste

 

[PlumbBobBob]

the basin mate will be fine then. they are a very tight fit so have to put some oomph into putting it on. once on will not leak.

 

[GQuigley67]

here's my 2ps worth

silicone bead on the top of the basin under the flange, make sure plenty comes out and wipe away excess.

Rubber washer on bottom if of good quality then mostly this is OK, if not then some silicone and should be fine.

Don't overtighten

And don't put the rubber ontop of the basin or the grating will be too proud and will have a puddle of water gather around the grating which won't be able to drain away.