you apply for the cashback offer when you register your items on the Nikon UK website
After you have done this you have to wait (about 10 days for me) until you receive a link and logon details to fill out an online cashback form
You need to fill out your bank details on this, including your IBAN No (International Bank Account No - My bank Barclays, has an online form for generating this)
Once this is filled out, you have to send a printed copy along with a copy of the warranty slip to Nikon.
I had to wait for approx 2 weeks until I recieved an email informing me my application was being dealt with - it took a further couple of days until the money was showing in my bank account.
All in all a couple of days short of a month - a bit of a long winded process, but I suppose thats all part of the cashback offer !
I wonder out of interest how many people don't bother claiming.
Hope this helps.