Sunset F

I have a limited company and have completed some work for an Italian company. I sent an invoice but the remittance notice has come through and they have retained 8% tax, which they say I can claim back under the double taxation treaty. However, before I took on the work I sent them a UK tax residency form (twice) and I also filled out their company form and got it signed by HMRC (twice) that confirmed my tax residency in the UK. The income goes into a business account so I don't declare it on a self assessment form but in the company accounts, so I don't understand a) why they have retained it b) if this is even legal c) how to get it back - can you help me? I feel like I have jumped through hoops for this company and still I'm not being paid properly.