Agency worker or self employed?

spinelli101

I work offshore, through an Agency (whichever agency offers me a job) who sends me to provide services to the Client. My contract is between myself and the Agency, and typically involves being on-call 24 hours a day for 5 weeks, when I live and work offshore. I realise I am definitely not 'employed'. I invoice the Agency at the end of the job (my day rate is mutually agreed upon in the contract) and sorting out my taxes is up to me. However, I don't know if I should be registering as self employed, or whether I fall under the Agency Workers Legislation (ITEPA 2003). This is important to me for the following reason.

As I work offshore, I may be eligible to Seafarer's Earnings Deductions (SED) depending on how much of the year I spend offshore. However; I am only eligible if I am employed.

According to the Agency Workers Legislation,
"services rendered by the worker are for Income Tax purposes treated as if they were the duties of an employment held by the worker... Accordingly, all employment rules apply."
Does this mean that if I fall under this category I should be eligible for SED?

If in fact I am self employed, can I legitimately start up a limited company, and pay myself dividends through that to reduce the amount of tax I pay?

It's a complicated scenario and while I want to lose as little of my earnings as possible, I don't want to do something wrong and get bitten on the !!! later!

Many thanks to anyone who can help, or point me towards someone who can (HMRC and various accountancy services have so far been useless )