Some thoughts...
Hello Graeme,
Very attractive watch, I can see why it caught your eye. Unfortunately to have a thorough discussion of the piece you'll have to open the case back. Once you've noted the serial numbers on the case back and movement, and have posted photos of both, a more complete assessment can be given. I will say that C83s with a gold Calatrava case are quite unusual and if the watch is in red gold as the photos suggest that would make it even scarcer. But I must also caution you that the dial has almost certainly had a repaint at some point in the past. And while that may or may not excessively affect it's worth, it could diminish it's value to a collector. It may interest you to see the a photo of the black version of the dial on a steel cased version I own.
Kind regards,
Michael
