Hi,
Thanks for your help on and off the forums... in case anyone wants to do the same sort of thing as me, here's the custom macro I eventually used:
case "v_lar":
match = true;
if (HttpContext.Current.Request.Cookies["UserSettings"] != null)
{result = HttpContext.Current.Request.Cookies["UserSettings"]["Large"].ToString();}
else {result = "false";}
break;
The cookie value is set as either true or false, making it suitable for a visibility property, and if no cookie is set, it also defaults to false.