The tyre shop took the wheel off the car and did a thorough check but failed to find anything. They slowly rotated the wheel whilst spraying liquid across the surface - I guess to see if any bubbles came through... They also checked the valve and it appeared to be ok.
This tyre shop has a good reputation..
I think I'll try somewhere else but ask them to remove the tyre if they can't find anything from the outside as it may be easier to detect from inside..
Or maybe over time it'll get worse enough for them to find the problem.