Just found the full Ts & Cs http://www.nissan.co.uk/GB/en/inside-nissan/innovation-and-technology/innovation-news/care.html
"If you can provide a written quote cheaper than an authorised Nissan dealership's quote for servicing and mechanical repair work to your vehicle, subject always to such competing quote being (i) inclusive only of Nissan genuine parts and accessories and (ii) from a recognised vehicle repair outlet within 10 miles of your local authorised Nissan dealership, we will carry out the servicing and mechanical repair work for the cost of the lower quote.
All competing quotes will need to be provided on a recognised vehicle repair outlet's headed paper. Nissan reserves the right at its discretion to accept or reject any competing quote if in Nissan's opinion such quote has not been provided from a recognised vehicle repair outlet."
So they want the other place to be using genuine Nissan Parts, which I suppose might be a bit of a pain.