Taff,
If you think you have left it too late to get out of the country, all might still not be lost.
If the majority of days that you have done out of the country have been relatively recent (say, for instance, in the last 6 months) then you can shift the date that you want to start claiming your SED closer to the beginning of this block of days, leaving you another 6 months to get the rest of your 183 days out of the country.
This means that you will have to pay tax on the earnings prior to the new start of your SED claim, but this will likely be at a lower rate because, the chances are, you will have worked fewer days during that period.
It might work out a lot cheaper this way (especially if you are staying in the maritime game).
I won't go into detail here, but something similar happened to me a few years ago and I had to pay some tax, so I've got some idea of what I'm talking about.
Can't make any promises mate, as I'm just about to deploy myself and will probably be out of comms, but if you send me all of your dates in and out of the country, and if I get time, I'll have a look at them and then maybe offer some advice (although you will need to go to a specialist maritime accountant to confirm anything I say).
I'll help if I can - good luck Royal.
.