In my reply to the Readers' Forum query Diving dilemma, I mentioned a pamphlet produced by HMRC entitled Notes for Guidance on Taxation of Divers and Diving Supervisors Working in the United Kingdom or UK Designated Areas.
This pamphlet is not available on the HMRC website, but I enclose a copy for your reference.
Editorial note: the pamphlet is available below as a PDF attachment.
Notes for Guidance on Taxation of Divers.PDF