Absolutely not! NEBOSHH might be useful, but is certainly not essential. All you need to expand your qualification is an operator certificate on the machine type concerned. Obviously, practical working experience also matters, the more the better. Remember, however, that to gain the support of one of the accrediting bodies and to enhance the attractiveness of your training operation, to work commercially, you should also be accredited. This is an annual chore, but is worthwhile.