If the handbrake mechanism is seized on then there shouldn't be much tension on the cable? the nipple should slip out easily enough? But if you struggle then try applying the handbrake fully, put a clamp on the opposite side to stop it releasing and take the handbrake off. That ought to release the cable enough to get it out.
Then it's a rebuild kit if you can still get them?
Thanks for the replies everyone.
Guy, the opposite is the case. Because the lever is seized in the on position the nipple is held firm into the socket. Possibly the cable is seized in its sleeve also. I'm going to attack it again later this week with the caliper fully removed. Don't follow your clamp theory (what sort of clamp?)?
I'm won't be refurbing the caliper myself, so any recommendations for firms doing this would be welcome, other than Big Redd, who unlike everyone else, I've now had two bad experiences with.