Thanks for your reply Simon, and everyone.
I've had a look at dates and there can't have been more than a month and a half between exposure and development of the attached picture, though the film had been in the camera for at least a month before that. All of the frames show the same grubbiness, even the last one taken just before development. I'm not sure this is a simple latency issue as it affects all exposures, not just the old ones. Storing in the camera for too long? Crap development?
I agree that developing films myself is the only way to be sure of what is going on, but I've found it best to send them off because [insert multiple reasons here, including time, chemicals going off, baby, time, baby, baby etc.]. Maybe next year...