Having installed acrylic baths of the 1980s and 1990s with the 'banjo' type plastic waste fitting, I soon learned that stains of the cieling cost money and eat profit. Best thing for a banjo type waste is the bin.
Hence, I have always fitted the best quality pop-up waste the customer can afford. These used to be branded as 'bette' wastes from the trusted German manufacturer, but bristan do one now which is the same. However, do not be fooled by things that look the same in pictures but are not.
See the diagram here [DLMURL]http://www.bristan.com/WebRoot/BristanDB/Shops/Bristan/Products/INSTRUC.%20PDF-1029039-D1.PDF[/DLMURL]
and note that item 7 is flange washer that fits both above and below the bath waste outlet. The washer has dual purpose and needs no silicone or sealant, and neither does the overflow.
Another advantage of this type of pop-up (usually betwen ÂŁ20-ÂŁ40) is that it can make a ÂŁ100 pressed steel bath look like a million dollars. They are neat, tidy and you won't have leaks on these.