I have loaned a colleague / friend £15526 over the last 3 years. I have asked her to start repaying it monthly, but i just get excuses. What's my best course of action?
The correct answer here depends upon a range of factors: the nature and importance of the relationship; the likelihood that the friend / colleague would respond positively to the different types of pressure you might exert; whether the friend / colleague has access to the funds to repay you; etc.
I can't really advise here without knowing a lot more - I suggest you speak to a local solicitor about this initially. Many solicitors offer an initial short consultation free of charge.