My lemon tree (in a pot) is about three years old. It has never fruited and I was advised I should feed it this summer. Since then the leaves have turned yellow. Can you advise?
That sounds like it could be chlorosis. It can be caused by something as simple as overwatering, or a pH imbalance in the potting mixture. See the Wiki which gives some causes and advice:
and some from the RHS:
All of the leaves turned yellow and fell off of my orange tree once but they grew back after I repotted it.