Custom link gets chopped off

Guys, |Can somebody please advise, I am using this line below as a Custom on my project, The idea is it will open the website and return the map to display the area by the Grid Reference.
The value in the Grid Ref is SJ 54456 85962

$value="<iframe src=http://gridreferencefinder.com/?gr=".$data["GridRef"] . " height='1000' width='1200'></iframe>";

But when the website runs only the link underlined below makes it to the SJ and the important bits at the end 54456 85962 get chopped off
Any ideas please how I can make the whole link complete please?
Thanks


<iframe src=http://gridreferencefinder.com/?gr=SJ 54456 85962 height='1000' width='1200'></iframe>