Replacement gasket arrived from Land Rover. Exactly the same as the one before i.e. for the other side. The parts man was very apologetic, took photos of the wrong item, the old one that came off and the roof rail itself. He said he will e-mail Land Rover and request a stock check. He'll let me know, but he's not holding his breath.
Worse case I can use the wrong one - remove the glue that is there somehow, turn it over and glue the other side with impact adhesive or something. I assume the glue is just there to stop the gasket moving during installation and serves no longer term purpose.
Oh yes, and the Sika Flex I bought the other day has a use by date of Jan 2021. I'm doomed at the moment.