London,
United Kingdom 
I have had an orange tre on my landing for the past 8 years; I feed it with baby bio, and up to now it has produced new leaves every year, and lots of oranges.
But this winter, the leaves have fallen and fallen, until what isleft is about three quarters twigs, and one quarter leaf...which of course indicates that it is on the verge of dying. On looking at" answers", and the advice a member gives of a possible nitrogen deficiency...what fertiliser, based on nitrogen, would you recommend; I also feel I should, after all this time, re-pot it, which I have never done.
15 Mar, 2016
Yep, you asked a similar question the other day, and definitely you need to pot into something bigger. Regarding fertiliser, you can buy specialist citrus fertiliser, but the RHS recommends lawn food, being high nitrogen. I would point out they just mean simple lawn feed, not a combined preparation with mosskiller or weedkiller in it. More info in the link below, though not all of it is relevant to your situation
https://www.rhs.org.uk/advice/grow-your-own/fruit/citrus?type=f
15 Mar, 2016