Thank you very much for your reply which has been very helpful and the rules are so confusing. However I note from the HMRC website the following:
An ‘eligible period’ is made up mainly of days when you are absent from the
UK. You are absent from the UK on a particular day if you are outside the
UK at midnight at the end of that day. Non-work days spent outside the UK
may be counted as days of absence. A return visit to the UK can also count
towards the eligible period if:
• no single return visit lasts for more than 183 consecutive days, and
• the total number of days you have spent in the UK is not more than
one-half of the total number of days from your first day abroad to the last
day of the period you spent abroad after that return visit.
Given that I comply with the above does this not make me eligible. This is from HELP SHEET 205, sea farers earnings deductions.