If the leaves look pretty green and healthy the tree probably just needs more regular watering. If it is citrus leaf miner, a moth that lays eggs in the leaves, the leaves will be quite deformed. - Isobel
Check also for insect pests, especially on the undersides of the leaves. A thorough application of white, eco or horticultural oil over upper and lower leaf surfaces and the bark may prove beneficial - it certainly won't cause any harm.
Consider your feeding regime too. Always use a citrus fertiliser that includes boosted magnesium and iron to assist leaf greening and strength.
If citrus trees are otherwise healthy, with green foliage, flowers and plenty of fruit ripening, then the most common cause of leaf curl is lack of moisture. While you may be watering periodically when it hasn't rained and the potting mix may feel moist, the trees are not getting sufficient moisture to continue maturing the fruit while maintaining their vigour.
During hot weather, potting mix even in large pots will become quite warm and this too inhibits moisture uptake and increases evaporation from the potting mix. The heat of the sun will also evaporate moisture from the leaves faster than the roots can take it up and circulate it through the plant.
Over summer it is essential to water deeply every couple of days, ensuring the excess moisture can flow freely out through the drainage holes in your containers so the plants don't suffer from 'wet feet'. Regular soaking not only keeps the potting mix moist but also cools it.
While citrus generally love heat, when grown in pots rather than the soil, they may benefit from a shade cloth cover on extremely hot and/or windy days, just to reduce transpiration (moisture loss via the leaves).
Citrus are best planted out in autumn or spring, so they have time to establish before being exposed to extreme weather. - Noelle
Citrus leaves can curl when a disease is present, temperatures are too cold or too hot, or an insect infestation such as scale, mealybug, mites or aphids. Over or under-watering can also cause citrus tree leaf curl.
Try opening some curled leaves to see if any insects are living inside them. Pest oil will take care of most insect infestations. Apart from the curled new growth, your citrus looks rather healthy and should only need some fertiliser every six months.
You should find this guide useful: Growing Citrus Plants.
I find some screen with coarse gravel or a broken brick layer on top works well for drainage hole coverage for my pots. I use a dedicated citrus mix like Scotts Osmocote 25L Citrus & Fruit Premium Potting Mix or a mid-range potting mix. These are an open mix which is free draining, and luckily enough, I haven't experienced ill effects from them compacting. Currently, I have an orange that I deep soak regularly planted in a three-year-old mix which has heavily compacted. The pot appears to drain adequately, and the plant isn't showing signs of leaf curl. I would encourage the use of an open potting mix and adequate drainage on any potted plant which receives regular deep soakings.
I suspect that pests have caused the curling of the leaves and not over or under-watering. Are there silvery tracks over them, or is there evidence of insects under or in the curled leaves? If there are silvery lines, then that would indicate the pest citrus leaf miner is present. It curls the leaves similar to what you have shown.
I'd suggest spraying the tree thoroughly every two weeks with Yates 750ml Ready To Use Pest Oil. You'll want to coat the top and bottom of all leaves with the Pest oil. This will rid the plant of a variety of pests, including the leaf miner. Fungal disease can be treated with Multicrop 30g Kocide Blue Extra Fungicide.
Alternatively, it could be citrus leaf miner, a tiny burrowing mite with the distinctive trait of causing silvery trails and twisted leaves. However, I do not see the distinctive silvery trails in your image. The infestation is found on fresh new leaves, which does appear to be the case here. Other insects which cause similar damage include scale, mealybug, mites or aphids. Spray the recent growth once a fortnight with Eco oil until the leaves have matured and turned a dark green colour. I will also encourage you to apply Scotts Osmocote 500g Citrus & Fruit Controlled Release Fertiliser if you haven't fertilised already. A well-nourished plant will happily bounce back from these insect attacks.
You might also like to take advantage of our perfect plant promise. All our plants are guaranteed for 12 months, so if you’re not 100% happy, return your plant (with receipt), and we’ll refund it. - MitchellMc
If the curling is because of lemon scab, get a copper spray to control this happening. - woodenwookie