Just to add/compliment to Jon's answer - it is legally very difficult/not possible to impose a form of contract, however you can "strongly encourage". If anything other than an NEC contract is proposed it has to be accepted by the Project Manager, so if I was a Contractor that would be one less acceptance I can avoid by using NEC down the line as well.
As a Contractor I can not think why I would not want to be anything other than back to back.
As a Contractor I can not think why I would not want to be anything other than back to back.