IANAL, but I think it's quite clear that this is the company's property.

You make a big thing that it was not done on company time, but that isn't really important. It was related to your work, and you used your experience from work to do it. You've also been using it at work - that implies that it was done for work. And then work asked you to make changes, which you did presumably without complaining. You've done everything to set up the image that it's the company's property.

Also, what does your contract say?

I think this is a pretty clear-cut case - sorry.