Apparently adding the following at the very end of my web config sorted my problem, why, when there was none originally set in the config, my maxJsonLength hard coded in the web method didn't work I have no idea, but never mind hey? :doh:
<system.web.extensions>
<scripting>
<webServices>
<jsonSerialization maxJsonLength="500000"/>
</webServices>
</scripting>
</system.web.extensions>