Hi Richard
What is the URL looks like on the page where you have the table view (from screen shot) from which you can add a new column (child)? From the way you extracting out the parameter, it looks like the original URL may contain the Source URL parameter you wanted already. %3F is URL encoded string for "?" and %26 is URL encoded for "&". I understand the outcome URL you would like when user clicks the New button but would like to see the original incoming URL that this custom action is trying to work with.