The vehicles had a 12 year chassis corrosion warranty.
If it's outside that, and Nissan have refused, then I don't see what you can do. I think it's highly unlikely that a court would rule in your favour if you brought a case against Nissan.
My my, we have come a long way. I can remember Ford P100s failing their 1st MOT on chassis corrosion and being written off as uneconomical to repair when they failed again in years 4 and 5.