lemon tree leaves sticky

Question

plant kept in conservatory facing south west

Answer 1

Most likely caused from a scale bug infestation.
Neem oil is good treatment but you can make a insecticidal soup also.
You may want to check all your plants and quarantine your Lemon Tree.

Here is a few links to help you.

https://www.gardeningknowhow.com/plant-problems/pests/insects/control-plant-scale.htm

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm