@Showtime, Business Details is where you enter things you want to show on every transaction document. Surely you do not list your accountant’s name on a sales invoice you send to your customer, do you? Nor would you list your directors. The purpose of the feature is not to store general business information.
Nevertheless, you can put as much information as you want in the Contact details field. And it will show below your trading name on documents, in the header. For that matter, you can also put additional information in the Business identifier field, separated by <br> tags. That also shows in the header.
Custom fields, on the other hand, will appear at the bottom of the document by default. Use them if you want other things. Set them as defaults under Form Defaults.
The bottom line is that what you want is already provided in the program via more than one route.