So, you are undertaking a contract with Modis, resident in Portugal and the work is being undertaken in Portugal.

- How are Modis paying you ? Are you invoicing from a Portuguese Lda having created one or already owning one ? If so then it is just trading income in Portugal and I am sure your Portuguese accountants will deal with it in accordance with the appropriate regulations and will of course know what techniques may minimise your taxation.

- Are you being paid PAYE ? In this case you need to try and arrange an NT tax code (which should be possible as non resident) and no tax will be deducted from the UK end of things. How this income is treated in Portugal is down to local regulation. But I rather imagine it will just count as "oither income" and be subject to tax and ss accordingly through your Portuguese tax return.

- If UK tax is deducted then you should be able to reclaim this or offset against any Portuguese tax via the provisions in the UK-Portugal tax treaty.

But it is not a simple area. You should get proper advice.