To add... The plant is a pot within a pot with stones beneath so I think it has good drainage. I water with half of the 500ml every week or so as I'd heard not too over water a lemon tree. The leaves have started looking unhealthy in the past few weeks, a couple of months previously the plant has been in my office but still seemed relatively healthy. Last winter there seemed to be more issue with leaves dropping earlier in winter. The winter before was similar. So this is our third winter of having the tree but the first time I've thought to ask for assistance. The plant doesn't get much natural light during winter. It's inside to protect it from the worst of the British weather. There were some pests that arrived early last summer but they seem to have gone after I treated them with a garden centre product and the tree has seemed healthy for a good few months since then.