When you say you have problems with the roof since owning the car that does seem to me that the 'fault' lies with the supplying dealer. NGB may say you go back to the supplying dealer, but they may be no better than Wilsons - frankly there seem to be too many Nissan dealerships who are not familiar with ZEDs, and as yet we have to hear of one that has got a suitably trained person to sort out roadster roofs.
What this is getting to, is that I expect NGB will want you to go back to 'your' dealer. But insist on being pointed to a dealer who has a suitably trained person (if there is one). Otherwise you will get the same problem as you found at Wilsons and Plymouth may be no better.
Alternatively, put your problem in writing to the MD at Plymouth on the basis you have been sold a faulty car and what are they going to do about it?
If NGB were more focused about customer care they would send a suitably qualified person to meet you at your dealership of choice to identify the problem area and tell the dealer what has to be done, and if necessary come back and verify the work before releasing the car to you.................oh oh - just remembered. pigs don't fly