Depends what job? Part-time at Tescos, nope, I wouldn't even think twice. Not only would I not be massively bothered about walking away from that job but I know that £1,000 will cause no harm to the company. If it was part-time at a family run cafe then yes, I would, as it would affect their business probably quite substantially.
However if it was my full-time graduate job that I intended to stay in and work my way up in, yes I would tell, because £1,000 < career.