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...