The seats had been looking tired for a while, and the dye had worn off in places (more than 5 years after the original job!)
I ordered a custom leather colouring kit from Furniture Clinic, using a swatch cut from the original S-Limited seats (this colour, which is the same as the LE Miata is now on Furniture Clinic's system). This is a water based dye; you prep and clean the leather (celluose thinners and alcohol), sponge on the dye, and then spray the top coats.
This first pass showed the change:
The finish was still very patchy, thanks to problems spraying the dye using the supplied airbrush. A better airbrush was secured.
Getting better
No attempt was made to fill the cracks, but the dye has made the leather look new
Still not quite perfect, but at this stage, I got bored and sealed it. Furniture Clinic sent me another batch of dye, enough to do 10 seats, so may have another go another year.