Some relevant points on this thread.
SC-130J Sea Hercules Maritime Patrol Aircraft - Page 2
It is not surprising that the MOD has yet to make a decision?
Cancelling the MRA4 may well have been a mistake with hindsight.
Whether or not an MPA will be needed until 5 years from now is another debate.
Then there are, as ever, political decisions to be made. The next election is scheduled for 2015.
No decision until after that date?
It is likely that the Tories want the Boeing P8. They would order it tomorrow, but there is opposition within their own party because of the impact on British industry and jobs.
Don't be surprised if they make a decision on the eve of an election. Especially if they believe they are going to lose.
Similarly, the Labour party if it came to power would also be worried about UK jobs.
If Labour became the next government, they would delay the decision or get Marshalls to convert a few C130Js, until Airbus came up with something firm.
In the end it will not be technical or military decision, but one that involved politics and jobs.