HMRC has published guidance on Dual Contract Arrangements.
Dual Contract Arrangements are most commonly seen where a foreign domiciled worker works both inside and outside the UK. S/he may claim to have two employment contracts, usually carefully drafted, and the bulk of income may be attributed to the contract which appears not to be subject to UK PAYE taxation. HMRC is sometimes suspicious of such arrangements and will investigate them.
The new guidance sets out the evidence which HMRC expects an employer to keep, and explains how it approaches the question of whether work in the UK is 'merely incidental' to employment outside the UK.