This sentence intrigues me. Do you mean you employed witter to specially design and make the tow bar for you, or do you mean its one Witter has designed for the car and is available to anyone?...
The towbar I was custom built by witter to fit my car.
...
The reason I ask, is that the UK law (which is still based on the EU construction and use regulations) say that tow bars and hitches must be type approved to be fitted.
I suspect you mean the second option becasue I doubt you would consider the cost of getting the required type approval for a "custom" (one off) would be practical.
I suspect your best approach is to focus on the tow bar and fitters.