Hi MonkeyHanger,
it is a good idea to get the NDA signed when approaching companies that have their own products or do design work etc, but once the design is our there then it'll be no use at all. also consider that as (i guess) a one product company you would be a small fish in a big pond. if your design is considered excellent then some of the bigger players might consider selling it under license as their own brand which might give you a route to get a patent by getting them to fund it with your percentage initially.
if you do decide to go it alone i know of a good company that largely specialises in injection moulding design and manufacture of their own products but they also take tooling work on. I mention them because they have their own factory in China for tooling and manufacturing which will help a hell of a lot with your costs and pricing. I have no affiliation with them other than using them as a supplier for our companies own mouldings. if you are serious about going it alone I can get, them to contact you

I can also give you some idea product costs etc if you need help.
another point to note is that most companies will take you more seriously if you have a working prototype, you can *******ize a current net and get your hinge system etc rapid tooled for a possibly a few hundred pounds if the design is already drawn up. either way let us now how you get on
Chris