Selling a property surveying application.

Hi. I run a small property surveying company. We had an application developed in-house to help when carrying out the surveys. Some of our clients have contacted us about licensing/buying the app so they can edit the surveys we send to them. I am UK registered, the app has been developed in English, however I believe some of the companies have staff throughout Europe who will likely be using the software. What are the regs on selling/licensing the application.