Should there be a PAYE deduction from my sole proprietor invoice?

I operate a sole proprietor providing services to various companies on an "in need" basis on short and medium tem basis. I operate at my client place of business however the latest client has started deducting PAYE from my invoice at a rate of 33. 3% although I am not on a fixed contract and will not derive more than 80% of my annual income from them. Are they entitled to do this and how do I reconcile these deductions as a provisional tax payer?

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