Hi Oli,
Why non-specific interaction happens we all want to know. If you can solve this you will be a rich man.
That said. You should go back to the original experiment and review the conditions, surface and proteins. Only then you can try to solve non-specific binding. The analysis that you present is not valid. There is a mathematical curve but this has nothing to do with proper kinetics.
Also look at
www.sprpages.nl/experiments/troubleshooting
kind regards
Arnoud