How much citrus food did you use, Jeany? Some brands have directions for trees in the ground--I guess they assume you live in Spain or Florida!--but not for pots. A dose for a tree in the ground can easily knock the fruit off a tree in a pot.

Hi Tugbrethil,

Thankyou for your reply. The pot is roughly 8"w x 9"d, the tree is 18"h x 26"w. I mix 1tsp of summer feed in 4 pints of water and feed it weekly. Is it normal for the fruit to be this dark colour to start then maybe change or do i have a problem, do you know? Thanks again.

Some varieties of lemon do start with a dark purplish green fruit, which then changes to deep green, until it is ripe. The way to tell if the baby fruit is alive or not is to look at the surface of the skin--glossy is good, but dull and shriveled is bad news.
That is an extremely small pot for the size of the tree, though, and I'd be willing to bet that it is suffering some drought stress between waterings, which will also kill the fruit. An upgrade is probably in order, though early spring, before bloom, would have been the best time.

Hi again Tugbrethil.

Thank you very much for taking time for another reply. well the good news is as you have explained the surface skin is glossy and not shrivelled, infact they are beginning to turn green!
I agree with you on the pot but not knowing anything about citrus tree's went along with the garden centre advice and didn't repot it. Can i just ask one more question please, when would be a good time to repot, as it now has a new lot of blossom, Autumn? worried it may not survive that long.

Thank you once again, your advice has been invaluable.

Well, if it is life or death, I would sacrifice this year's fruit, and go ahead and repot it now. Use a fast-draining, neutral to slightly acid compost, and feed it half strength at it's first watering. If a lot of soil falls off the roots, prune the top enough to remove about 1/4 to 1/3 of the foliage. Since it is blooming multiple times, sounds like a 'Eureka' or 'Meyer Improved' Lemon.

Sorry for my late reply. Well from looking very sorry for it's self, it has picked up and is covered in blossom. I have watered it more and not allowed it to dry out in it's small pot. Thank you so much for your continued help. I am going to do exactly as you say but as it is happy at the moment, will watch it and repot as you have suggested in the Spring. I will also try to find out it's type now you have narrowed it down for me. Again many thanks for your help.