Paul,
1. How you make a prototype will depend on the job its going to do – a spanner and a screwdriver are not necessarily going to be the same; moving parts like pliers would be a difference again. If the material needs to be hardened etc, or if for a proto it doesn’t matter. Then if you are going to make lots – it needs to be designed for manufacture.
2. Even if its the best thing since the molegrip you will need a plan to commercialise it and exploit it if you will make any money from it. You might need prototypes to test this – but if you don’t have contacts it can be a real uphill struggle.
3. Most machine shops doing specialist work will be used to signing non-disclosure agreements. This is quite normal. As will any credible designer who will commit your thoughts to paper.
4. If the function is really clever it might actually be more appropriate to file a Patent rather than a registered design. A registerred design will not protect the function only the ‘form’, so I could take your concept and produce something that does the same job but which visually is a bit different and I won’t be infringing your rights.
5. It will take months as a minimum for the Patent Office to respond to your application. Even then it won’t really tell you how protectable your idea is, or that someone can’t walk all over what you’ve written. Patents are one of the areas where amateurs CAN do there own filings, but if you ever needed it you’ll almost certainly have missed things or left gaps.