Hello and welcome to the site. Thank you for your question.There are conditions set by HMRC that have to be met in order for Per Diem allowance to be tax free. It is important that such arrangements are made with HMRC's approval, otherwise they would be considered as simply "Round Sum Allowance"and processed as additional salary and subject to PAYE tax and NIC.More information on this topic can be found here
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Thx for your answer does that mean I can claim per diems as per the world wide persistence list? and hence change my existing Self declaration?