JPAC have deducted tax and NI at source, and while you could recover the excess tax deducted by JPAC by completing a self-assessment form and submitting your P60 (not issued until April 09) to show that you'd been over-taxed, you can not recover the NI in the same way. It would mean that you'd be giving HMG an interest-free loan for the period from the full amount being deducted/paid until the refund of the tax comes through, and I can't see why you should suffer because of their mistake.
Since it's JPAC's fault, I suggest you ask JPAC to sort it out. They can if they want to, but it probably will make their heads hurt.
In the mean time, write a cheque for the net pay you've been overpaid, and post it by recorded delivery to JPAC, with a letter explaining. That way they can't moan that you've done nothing, and I can't see how they could possibly take any action against you.