Hi Alice, we need much better photographs of the leaves that are curling. Picture of different angles and of the whole tree. Can't diagnose what I can't see but making a stab in the dark my first reaction was leaf miner. Can't confirm anything without better pictures.

I came home tonight to find my mandarin orange has now started doing the same thing!!!

It could be glare but here where I think I see leaf miner.

https://www.gardeningknowhow.com/edible/fruits/citrus/curling-citrus-leaves.htm

Thank you for the close-up. Yes, the curl is caused by leaf miner.

*Citrus Leafminer-(phyllocnistis citrella)

*larva of the silvery moth

*larva tunnel in immature leaves

*moths can fly 5 km per night

*15 km range in wind

*they appear with new growth

*As leaves mature they lose interest

*Leaves curl up at the edges when the larvae form pupae

*On mature trees, they are kept low by natural enemies

*On young trees maintain continuous light oil cover o the new growth which inhibits moths from laying eggs into the leaves.

*In Australia they use eco-oil (at 40ml per 10 liters) or pestOil at (50 ml per 10 liters)

I don't use oils so maybe someone else can provide a suggestion on brand and dose.

Oils do not really work on leaf miners. All they did was to roast the leaves.

The magic bullet is spinosad.

In Australia a company called nature way makes a fruit fly repellent. I forgot the name but it says spinosad as one of the ingredients. In fact anything containing spinosad works.

Unfortunately for OP it is too late. The mode of action of spinosad is to be absorbed in the leaves. When the larva eats it it dies there.

The wilted leaves will still function though. They can be left on.

OP can remove damaged parts and spray every three days once new growth appears.