Just to add to the confusion on this matter
The taxman recently- as in the last couple of weeks- decided that OK, as an agency PAYE bod, you were an employee of the agency.
However…..
The taxman also decided that they will treat each trip offshore as a seperate employment, meaning you need a foreign port call on EACH and EVERY trip to qualify for the Seamans Earning Deduction.
This is in regard to a case from several years ago, but they will no doubt apply it any and all future claims.