Honestly, talk to an experienced litigation lawyer, yes it might cost you some money but it can potentially save an awful lot of grief.

Trying to handle things yourself and being "reasonable" probably won't work - I know from a nasty experience when I was a start up founder and a customer who I was very friendly with turned round one day and sued us over an IP issue - probably the most stressful experience I've ever had.

NB If you are in Scotland I can probably recommend a suitable lawyer if you want.