Was this car an export car or was it purchased in the US and later shipped to the UK? The GM export rear lamps had the amber turn signal lamp incorporated into the lower portion of the tail lamp assembly and not a separate lamp housing.
Does the RPO sticker under the console lid show RPO CU9 (Delivery UK), or RPO EXP (export vehicle)??
The C4 convertible 3rd brake light was about that size, but it did not have the surround that's shown in the pic. There is also some lettering and something like a logo to the left center (highlighted by wax or polish??). That would not appear on the GM 3rd brake light. The only lettering theGM lens had was along the bottom of the lens and centered.