Well, things are slightly clearer than earlier this afternoon, but not by much.
If N&P are acklowledging they owe you £2400 in mis-sold PPI, it will be made up using the approach shown in this article by the Financial Ombudsman
http://www.financial-ombudsman.org.uk/publications/technical_notes/ppi/redress.html
Suggest you look at the 'single premium PPI' bit, which is most likely what you had.
The organisation should give you a breakdown of what it is made-up of, however the calculation should address:
treating the money you paid for the PPI as if it had instead gone towards paying off the loan
calculating how much extra you ended up paying, because you kept paying them money when the loan would have been paid-off, had it gone towards the loan and not the PPI
interest at 8% on the extra you paid
something about 'distress and inconvenience' payments.
Does that come somewhere close?
interest at 8% on the money you've paid and shouldn't