The leaves on my lemon tree are covered in a sticky liquid which also covers the floor under the plant. Is this normal? There were a few black flies and thrips which I have removed physically. Otherwise, the tree seems very healthy. The plant is kept in my conservatory.
It sounds like you have a pest problem, possibly other than the thrips you already removed -- sticky liquid is usually the honeydew secreted by any number of pests.
This article may help you correct the issue:
And this article provides additional information about common Lemon Tree problems: