This is common for citrus plants and usually nothing to worry about and It's probably still healthy. The main reason they shed their leaves is because they are stressed. This could be caused by over/under watering, changes in temperature, poor light or lack of humidity. I see that you're in the UK? So I would think that the reason it's loosing leaves is because of the winter temperature and lots of rain that we have been having.

I'm surprised that it has survived outside at all over here, let alone keeping some of it's leaves.

Looks more like a European garden rather than here in the UK.

Thanks. The tree is in Spain not UK as rightly noticed, so temperature is milder than UK. Trees nearby are in full leaf!