I was bullied by my supervisor and became really depressed and abandoned my PhD - making me even more depressed, Then I was told I had to repay my stipend? They are still pursuing me after 5 years. Do I have to repay it? Anybody else been in a similar situation?

It depends on the specific contract I assume.

In my department, people who quit repay only the part of the stipend they haven't worked for. So, say that you received 3000 pounds for 3 months of work, but you quit after one month, you need to give back 2000. Hope that makes sense.