A quick followup on the outcome of this Rover Colt 8.
Well, it turns out it wasn't a seized engine after all. Once I started to disassemble the engine it became evident that it was a failed starter, where the pinion gear had jammed up against the ring gear on the flywheel. It was jammed hard enough to prevent the pull starter from budging.
The replacement Bendix bits for the starter are on the way, but it runs now with a pull start - winning !.
I'm so glad that it was something simple, because I struggled to find a replacement Briggs that would fit into the narrow space at the back.
