There could be multiple reasons. Maybe the roots have run out of room in the container. That would slow down growth, if more roots cannot sustain more growth on the top.

Heat is not always a good thing for citrus. Too much intense sunlight and heat could be slowing down growth, especially for a plant growing in a container (having to do with less root space). If you're living in the UK, that suggests that too much heat and intense sun is likely not the issue, but it is still not impossible.

One smaller factor is that after a small plant produces fruit, its growth may stall for a short period of time after.

Thank you. The roots aren't trapped as I've had a look. It's a dwarf variety. I think it was over watered as I mulched it in a period of heavy rain. It's growing back now as we go into autumn.