Bought item 10374 from BES:

http://www.bes.co.uk/products/126.asp

and it comes with a black rubber sealing gasket (not clearly shown in the illustration).

Should this go between the upper part of the chrome fitting and the top of the shower tray

OR

below clamping between the underside and the back nut?

Currently have it on the underneath the tray, but am looking at the finish of the tray on the underside and it is rough as a'holes! Tis a stone resin jobbie.

Hope you guys can help?