At the time ( which was a about 4 years ago ) the Marlow D12 was about the only real high strength available, but in theory absoluteley fitted the brief. I used 4mm all around but in hindsight I should have gone down a size to 3mm which may have stopped some of the creep. I notice to that you can get this D12 now in pre streteched form so maybe it is worth a go.

D12 is one of those ropes I find totally indespensible, you can do almost everything on a boat with it, from soft shackles to main sheet ( in the cascade system ) to the spinny retrieve where it is soft enough that it never seems to burn the spinny no matter how hard you try.