Thanks for your advice.
When I finally separated the arm from the caliper, I found that the pin was also rusty, which is the likely cause of it seizing, rather than the deflection of the pin. It's more or less straight, now and cleaned up, but the coating has gone, due to the rust. It seems ok, so I'm gonna run with it and keep an eye on it.
in the meantime ... if anyone were to wander by, that's done a WP upgrade and has a right hand caliper that they weren't using, then I'd be very interested!
Thanks