How can you tell when you count as employed for tax, or other, purposes?
Is it better to insist on being paid gross, and how do you go about that?
How can you lose out if you let PAYE deductions serve as a "piggy
bank" for tax? The articles linked below - all from the Fees
Guide - give general advice, roughly in order of increasing nit-pickingness.
You can do quite a lot yourself to encourage clients to pay up on time. Invoicing them for interest and compensation often wakes the accounts department from its evasive doze. The links below explain how - and, for NUJ members, how to get the union to help if they're really recalictrant.