P45 Legal Obligations

[Biker 1]

If someone leaves a job 'under a cloud', & the previous employer is dragging his feet/not playing ball, what are the legal implications of obtaining one's P45 from him? Is there a maximum time frame in which he is legally obliged to provide a P45? If he doesn't comply, can HMRC issue a substitute/replacement?? The issue would be the nuisance of emergency tax in future employment....