Bath overflow fitting sizes?

[NeilPalfreyman]

I want to replace the chrome overflow fitting on my bath with a shiny new one and thought it would be really easy, so I bought this:

http://www.screwfix.com/p/bath-waste-with-chrome-plug-16-chain/32944

only to find that the plastic pipe that goes into the waste is a smaller diameter and wont fit on the new overflow.

Next I thought I would use the old back-plate and pipe fitting but found that the thread size and pitch are different to the new one. Diameter is smaller and there are less turns per inch.

So my question is are there "standard" sizes for bath wastes? The bath is an old enamel cast iron one so I guess it may have old imperial fittings?

Any idea where I can get one that will fit?

Alternatively, can I get an adapter for the plastic pipe? Needs to come down from 24mm (internal) to 20mm (internal.)

 

[NeilPalfreyman]

I answered my own question with a quick trip to the local plumber's merchant:

They had a McAlpine set which has completely different overflow fitting and pipe sizes to the Screwfix one and luckily one was compatible.