Lemon Tree/bush Leaf Problems

I have a small lemon tree/bush that is kept in the conservatory, it is watered regularly and it can get quite hot as it's south facing. 

Towards the end of the summer last year the leafs started curling, I looked online to see what it could be but I couldn't find a definitive answer.

During the winter a number of leaves dropped off and the plant looked some what naked. The past 3 weeks the plant is going into over drive with new leaves which is brilliant. The only problem are the leaves are curling again. There are some pictures below of the new growth and old leaves.

I've also been misting the leaves the past couple of weeks to see if that helps.

 If anyone could shed some light on what the problem may be that would be amazing. If you need any more info or pictures please let me know.

Thanks!