Site vsitors can currently pick a radius from the drop down menu, you currently only have two radii options, 20 and 50…I use about 5 to choose from on my site . In the back end , put parentheses around the one I want to be the default. I usually have 10 as default . I would have more miles in the drop down to choose from.. 5 ,(10),20,50,100,200… I noticed you have like 190 in one cluster..you wouldn’t want 190 markers crammed into a 20 mile square radius. (I like your site btw.) http://locl.org/oxfordshire/map/
If something like that gets into play it will probably be part of the MY Store Locator Plus project.