Please see additional information added to this question. Original question : "Is there a plug for use in a shower tray. It would have to sit on top of the waste outlet which is flush with the base of the tray, so preumably it would need to be heavy and have a seal ring on its underside".
I omitted to mention that this plug is needed for a shower waste that,for some reason, has been fitted to a bathtub- which does have an overflow fitted. I hope I don't have to replace the existing waste system (which is awkward to get at) - if I can find a plug that will work with the existing waste.