[ausvette]

 remove <rule filetypes="mkv" player="dsplayer"> from the code and try that. i put this in as a test... but it was worth trying just in case.
dsplayer is the default player in kodi-ds so should play all files except those in the code playercorefactory code rules.

also remove one of the </rule> at the end

pay close attention to <rule filename=".*3D.*" player="MPC"/> the filename rule must start with a DOT then *3D.*
if you dont add the DOT then kodi will play all finenames with mpc as you are starting with a * (wildcard) which is all your filenames.

your code should now be:

<playercorefactory>
 <players>
<player name="MPC" type="ExternalPlayer" audio="false" video="true">
       <filename>C:\Users\Dave\AppData\Roaming\Kodi\userdata\Play3D.bat</filename>
       <args>"{1}"</args>
       <hidekodi>true</hidekodi>
       <hideconsole>true</hideconsole>
       <forceontop>false</forceontop>
       <playcountminimumtime>1140</playcountminimumtime>
    </player>
    </players>
 <rules action="prepend">
   <rule filetypes="iso" player="MPC">
  <rule filename=".*3D.*" player="MPC"/>
 </rule>
 </rules>
</playercorefactory> 

Thanks so much for your help and persistence with this.

With this, kodi is playing everything with dsplayer except the iso files which are playing with mpc.

Nearly all my 3d movies are iso's so that's great progress.

My 3d mkv files are still playing with dsplayer. My filenames are of the form "movie title 3D.mkv" 

Do I need to rename these to "movie title.3D.mkv" to match the rule?

Thanks!