The SaLT Team I worked with did some research with people with learning disabilities a few years ago which also suggested Century Gothic was the favourite (particularly because of the ‘single storey’ ‘a’ and lack of serifs). The only down side to that font is the question mark which is less clear, so we ended up substituting a question mark from Verdana or Calibri (a bit of an irritating process). If anyone knows of a font with a single story a and a sensible question mark, would love to hear of it!