Reneging FT offer in UK: Legal issues?



I signed a contract for an FT offer in an UK-based investment bank after finishing a summer internship. However, I decided to stay in my home country (not UK) and pursue a job in a different industry. As for the procedure I was planning to contact HR and my manager by phone to explain the situation. Thereafter I would send a formal resignation letter.

I am not sure about any legal issues that may arise. I received a sign-up bonus upfront, which I will obviously have to pay back. Moreover, I will have to consider the 4 week notice period, which also does not constitue a problem, as I was only supposed to begin in July '11. However, as I am not familiar with the UK labor law, I was wondering if I have to keep in mind any consequences this might have (compensation costs for additional recruiting etc.)? I am happy about any advice.

Many thanks.