The window seals arrived. But despite staring at the parts for a while, it was not clear how to remove the old seals and install the new ones without removing the glass from the car.
Having removed the drivers side rear window - quite an involved job on the cabriolet, it became immediately obvious that the old seal could be carefully pried out. Then, using a soapy solution as a lubricant - the new seal slid into the metal channel from the top of the glass.
So, the drivers side took several hours, the passenger side less than 10 mins.
During this job, various bits of foam insulation disintegrated on contact - including a foam strip stuck to the bottom of the glass. I found that closed cell pipe insulation cut to size works well as a replacement.