Dying Zanthoxylum bonsai?

Hi all, newbie to the forums but I've only just signed up because I think my poor bonsai is doomed

I bought this lovely little bonsai about 4 months ago and all was well until about 1 month ago when it came down with spidermite. I've been treating it as instructed by the internet for 3 weeks but now I have finally vanquished the spidermite, something else appears to have set in (my initial thought was blackspot but I have been treating this for a week and the leaf drop is only getting worse so I might be wrong?) and my poor tree has been losing leaves like mad!

Attatched are photos of the dropped leaves, the bonsai when I first bought it and then what the bonsai looks like now.

PLEASE if anyone can tell me what is wrong with the tree or if there is anything I can do to increase it's chances of survival your help would be much appriciated!

The tree is currently situated on a south facing windowsil in my bedroom in the North of the UK away from my other plants to prevent cross contamination. I water it whenever the topsoil feels dry, and until recently the bonsai had been fine with this arrangement.

Thankyou!!