According to the Watchdog site:
Is there a time limit for making a claim?
You must complain to the ombudsman within six years of the event complained about, or three years after you first became aware that you may have grounds for complaint, whichever is longest. For example, if you were sold PPI in 2007, you may still be able to take your complaint to the Financial Ombudsman Service if you only became aware of the possible mis-selling last year.
If your PPI policy has expired, you may still be able to claim, as the provider still may have broken the rules on mis-selling.
Are PPI policies still available to purchase?
The sale of PPI policies are predicted to shrink dramatically; some banks have even stopped selling them altogether. Banks and lenders who want to continue selling PPI will have to abide by the FSA’s new rules, thereby being much more careful about how they sell them to borrowers. Separately, the Competition Commission is bringing new rules that will stop lenders selling PPI at the point where they grant a loan.