HMRC's website http://www.hmrc.gov.uk/employers/p45-employee-leaves.htm has a faq:
Q. Do I need to issue a P45 to every employee who leaves?
A. You must complete a P45 for any employee who is leaving your employment and for whom you have completed a P11 deductions working sheet in the current tax year. You have a legal obligation to do this. The exception to this is when you have only used tax code 'NI' (used when a student is signing a P38(s) declaration).
I take that to mean if you have an employee (e.g. casual) who has not worked in the current tax year, then there is no need to issue them with a P45.
My payroll software does not allow 'nil' P45s to be filed online, so up to now I've been manually entering them on HMRC's website (!), which now seems unncessary.
My question is what do other people do in this scenario? Surely it's better if HMRC know the person is no longer on your books, so they don't send any more tax codes etc. The employer helpline says they will pick this up when the employee starts elsewhere.
Also, the point at which the employee is 'off the books' in the abscence of a leaver P45 is a bit vague - presumably employment law would apply, and the employer best send the (casual) employee a letter.