Hi - I live in the UK and I have had this serissa for just over a year. All was well until about 6 weeks ago when the leaves began to fall off and I noticed black marks on the trunk (pictures attached). I’ve not changed the watering regime but I have moved the tree, which is kept indoors in winter, out of direct sunlight. I’ve also sprayed with a fungal spray, but nothing seems to help.

I would really appreciate any tips.

I think you're too late. Serissa's do not recover from this.