why a blurry pic of a single leaf and not the entire tree in its conditions....

Good point, this didn't upload earlier

Tell us about the care you have given and its environment. BTW, citrus leaf miner does not cause sticky leaves. It tunnels within the leaf. Sticky residue is from sucking insects like aphids, white flies, etc.

Look on the under side of the leaves; it looks very much like you have scale.

I agree with John and Denise, spray your trees with horticultural oil or neem all over, especially underside of each leaf and the top of the soil. In my experience scale is very hard to get rid off, need 3 treatments at least.

Thanks Denise, John and Sunshine! Environment: north facing balcony though it's winter and the sun has dropped off. Water once a week with Seasol. Thanks for headsup re scale. How long between treatments should I leave it - just soaked

it in white oil? Shot of underside of leaf that's just dropped off (down to the last three though new ones are budding through) - Mark

What type of soil is it planted in? If the tree is facing north and it's winter, it doesn't need a lot of water depending on the potting medium. I highly suggest giving it more light because it isn't going to grow more leaves without the UV rays.

Denise - if it is winter where the poster is from, then they are below the equator (possibly Australia) and "North" there = "South" here, so the opposite (and very sunny although less so in "winter" than their "summer")!