Turns out, B&Q are still selling car parts.
I found this:
And turned it into this:
I started by cutting a hole in the chromed plastic plug that fits in the bottom of the light pull:
This slips on to the lock handle:
I then cut the light pull down to 40mm, and drilled and tapped two 4mm holes for some grub screws:
Then they get slipped on to the door lock, onto the collar and the screws tightened.
I'm really please how they've turned out - all I need to do now is paint the lock arm black so they match. For some reason when I got my bits from Marlin many moons ago, they sent one plated in silver, and another in passivated yellow.