Please help my lemon tree - leaves are falling off

Looks like chlorosis - lack of chlorophyll in the leaves. This has many causes and if left uncorrected, can result in the death of the plant.
Here is a link that might help -
http://www.urbanext.uiuc.edu/focus/chlorosis.html

Hope this helps

Are you feeding it?

Thanks maple and ukslim, I had a look at that article and think that it is definately down to lack of nutrients so I am popping out for some feed later!

Also look out for scale insect - they are tiny round 'scales' on the underside of leaves, usually by the midrib. Lemons are often affected. squish em with a nail or brush with a cotton bud dipped in 50:50 paraffin and water.
Lemons are quite heavy feeders, i feed mine Vitax twice a week throughout spring and summer, once a week in winter (its an all-season type) and also change the compost each year. I use 50:50 loam and multipurpose. Also i pick off lots of the tiny fruit - to leave one or two on a spur, this way i get 3-4 lemons ripe at a time and they are big!

thank you hoya - I will start being much nicer to my lemons! Brilliant tips...

I do not feed any of my plants anything. Just watering them only and lemons are hardy plants. They do not have to be watered daily. In fact, over watering and too much shade, could make leaves turn yellow and fall off since they love the sun.

If there is a disease, snipping off the affected parts is the best. Using pesticides specially on food crops is not that good. If it is a necessity, please look for harmless to humans but very effective on insects, Neem products. You will see a Neem tree on my page. This is not an advertisement. Neem Tree is the best health giving tree in the world according to experts. Neem seeds are used to make insecticides. Every part of the tree is used for herbal medicines. Hope this information is useful.