the span tag validation is occuring because it is part of a javascript:
infowindowtext: '<span style="font: 12px Verdana, Arial, Helvetica, sans-serif; color: black;">2a Cricket Hill<br />Finchampstead<br />Berkshire<br />RG40 3TN</span>',
the validator does not think this is valid HTML even though it is. This is a false alert.