Are you watering it with soft water ... I know much of Scotland has acid conditions, but lemons don't do well if watered with hard water. We always have to use rainwater as we live in a limestone area.
If that's not the case where you are, then it could simply be that the tree is suffering from irregular watering. It's a fallacy that citrus fruits should be watered sparingly, except for the winter dormant period, and they don't like drying out and then being flooded in summer.
Also use a proprietary citrus feed.

Lemons in particular are very sensitive to lack of nitrogen, and easily turn yellow and drop leaves if not fed regularly. We have no trouble watering with hard water here in the desert, but we do use acid-forming fertilizers on them--of course, rainwater is always better, if you have it. As for watering frequency, don't let more than the top cm dry out between waterings in a container.