Sounds like a cracked top lever spring, just replaced one on a pre-war Purdey today. A left of center but tight lockup suggests a sliding lock/bite that is worn to the end of its window. I’d hardly say this isn’t a big deal in the near future…

Purdey/Beasley can be a bit deceptive regarding checking for an off-face condition by removing the forend and shaking things due to its design.