My lemon tree is a mature tree full of fruit ,the fruit this winter has been rotting on tree ,we have had enormous rains all winter and gooing into spring is this the problem as hasn't happened before

Lemon trees are extremely sensitive to over watering, only thing you can do is make sure you have good drainage. Dig channels at low points to make sure ground water moves away. Just a small point, you posted your question on the Cars & Trucks section, you may be missing some better solutions by posting here, however, I've raised a few lemon trees in Spain and in my front room in London so I have a little experience in this area.
Good luck