How do I change "you do not have permission to view this content" message?

I have made a number of pages and posts available to subscribers only. I am using the Members plugin and Register Plus Redux plugin. This is all working really well as far as access goes, but I would like to change the message that unregistered viewers see when they try to access members only content.

The current message reads "Sorry, but you do not have permission to view this content"

I would like to replace it with something along the lines of "Sorry, but you must be logged in to see this page. You can sign up here (link to registration form) – all are welcome and membership is free. We hope you join us!"

For about a month I have been searching files to try to find where the "sorry" message is stored. I thought it might be fairly simple to replace - but I can't find it anywhere!

I'll admit I know very little about the back end of creating websites, so simple instructions would be greatly appreciated.

Many thanks, and fingers crossed!