Lemon tree

Its normal for citrus to drop a lot of the fruit, but i dont think its just nornal fruit drop looking at the fruit and the leaves

It looks like a nutrient defficiency, what type of fertiliser do you use? , looking at it it could be manganese defficiency , citrus need a lot of npk fertiliser and are very sensitive to levels of micro nutrients they use a lot of magnesium and others for the fruit

What temperatures is it kept at in the room? , how often/how do you water it?, do you have a gas fire near it?

From the leaves, apart from the nutrient defficency that could also be due to the plants roots being unhappy , the way the leaves are hanging....it looks a bit unhappy, have you got a thermometer that you can measure the temperature of the compost in the pot? ,

If the compost temperature is too low the plant will just hibernate and the leaves will hang like that as the roots do little work at low temperatures 70F to 90F is the best range for the roots for maximum growth , heating the room will do nothing to change the temperature of the roots a pet bed heater or similar works well

i would spray the leaves with a summer citrus fertiliser such as chempak summer a dilute solution, once a day for a few days, then once a week, also a once a week spray on a mix of potassium nitrate ( saltpeter ) ( 1 teaspoon to a litre of water ) and the same with epsom salts once a week for a month or so , you can get potassium nitrate on ebay and other places for meat preserving, a small pinch of sugar in the sprays will also feed the plant in the winter as theres not enough sun ( direct sun is bad in the winter due to cold roots )

Even if there is no defficiency, spraying with pottassium nitrate or urea ( industrial chemical.... ) solution a few times from a few weeks before the time you think it is going to flower will increase flowers and number of fruit that holds on the tree, pottassium nitrate is better as it doesnt have problems with impure salts like urea does

What type of compost is it in?

I am no expert on citrus but the leaves seem very yellow could you be over watering and are you giving it citrus feed? The plant also looks quite young so I would be tempted to not let it fruit yet anyway as the weight of the fruit could warp/damage the branches.

Could I also ask what the blossom smells like because mine has just come into blossom and its disgusting and musty like old feet lol I am wondering if it's just mine that reeks?

Oops took me to long to write it... good info above though really interesting

Thanks both of you ..

The temperature is never lower than 10 degrees, it's not near a gas fire, and I tend to water it once a fortnight in winter, more in summer.

It has a liquid citrus feed once a month.

I will measure the temp in the compost. I have read that they shouldn't really be repotted too often - it's in the pot I bought it in three years ago. Should I repot it this summer?

I'll spray the leaves too - I wouldn't have thought of that, so thanks.

The blossom smells very sweet. It could be just yours that has a unique smell!

The problem with most citrus plants is that the potting mix used isnt very good, it obviously depends on the supplier of the plant, make sure you let it dry out for a couple of inches down the pot between waterings, never water unless its needed as citrus roots are very sensitive to damage by overwatering.
If the room temperature is at 10 degrees the compost will be much cooler so root activity will be low and it will need very infrequent watering.
the minimum temperature that citrus roots will grow is 54F ( 12C ) and the optimum temperature is about 79F (26C), at low root temperatures the plant will wilt slightly looking like it is deprived of water
i think you need to warm the roots up slightly using a small heater mat such as a pet bed heater
Dont repot until the plant has grown new growth, wait for the new growth to slow and then repot, citrus grows in stages of root then shoots so repoting after shoot growth will mean the roots have somewhere to grow and they will grow rapidly into the new potting mix