In reply to Scarab9:
If you're generally a contractor, which probably means you're a limited company, you need to split the layers.
Your company (or you as a sole trader, but this can lead to problems if you work in the same place a lot, as you can be considered an employee) invoices the customer for work done. This is a business-business transaction and there are no employment rights associated to it whatsoever.
You are employed by and paid by your company, from which you probably draw a wage or salary. You are entitled to such things as leave.
So you are responsible to give *yourself* 28 days annual leave pro-rata, but you have to cover the cost of this from your company's income. Similarly you would, if sick, be responsible for paying yourself statutory sick pay.
So no, you can't bill any days other than those actually worked. It doesn't matter why they weren't worked.
Neil