I've been to the Osteopath now, and all is now clear.
What's happened is that I have a tendency to pull the muscles in my neck, it's a medical condition lots of blokes get apparently.
This I knew because I'm always putting my neck out of place.
The reason for the shoulder pain is thus:
My neck went on wednesday.
On thursday, after 3 days off with no car I worked flat out with a bad neck.
This inflammed the muscles and trapped a nerve, which has endings in the shoulder.
After manipulation in all directions he's satisfied the nerve is not trapped now, and my joints are fine.
The pain is due to the nerve being damaged.
All I can do is take it steady and wait for it to ease.
But don't throw myself back into work too hard.
There you go, just needed to see a professional!