Kitchen Mixer Tap (Kendal LA9)

Chrome effect kitchen mixer tap. The swan neck bit that goes into the horizontal bar needs a new washer, but not sure what sort (I threw the old one away because I assumed these things are standard, but apparently not). The tap doesn't have a brand name on it so can't check what type it is. Someone who knows a bit about this stuff might be able to figure it out, but when I put a normal o-ring washer on it still leaked slightly.