JACKO
From your statements, you are running a successful operation and to take a partner on board, is fraught with danger !!!!!!!!!!!!!
The classic response, you will get from all marketing and business pro's will be..................increase your charges, and stay in control.
The only reason for taking on a partner, should be.................to benefit you !!!!!!!!!!!
YOU, have built the business and YOU are the one who stands to lose out, if things don't work out and in partnerships, it's very rare, to find a truly 50 / 50 working relationship.
However, none of us know your potential partner.
Finally................how do those closest to you feel about the proposal, because, in the event of the partnership failing to shine, the repercussions spill over, into the family.
All of this sounds negative, but, it's what your bank manager / your accountant / your family will be asking.
Whatever you do, I wish you the best of luck and congratulate you on your achievements to date, you appear to have a successful formula.