diff_drive skid steer, front wheels not turning/veering right

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Hi, I'm creating a URDF for my robot which is tracked and I'm trying to use the diff_drive plugin in gazebo to simulate it. I can control the robot using the keyboard teleop utility but it veers to the right and when I look at the wheels in Gazebo I can see that though the rear wheels are turning as expected the front wheels aren't, if anything they look like they're digging in. If I drive in reverse the opposite happens.

The joints are all set to be continuous rather than revolute with a limit, I just can't figure this out and feel I'm bashing my head against the wall!

Is the wheel not turning expected behaviour for diff_drive with skid steer emulation or have I messed up with the URDF?

This is the first URDF and gazebo simulation that I've created so if I'm honest I've been doing a lot of trial and error to try and figure out why this isn't working and I've honestly lost track a little.

Full code is available on github if anyone wanted to give it a go themselves. I'm honest completely lost and no idea what to try next so if anyone could assist I would be massively grateful!

This is the definition of the right wheels:

  <link name="rear_right_wheel">
    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0" />
      <mass value="0.001" />
      <inertia ixx="0.000283500005025417" ixy="0" ixz="0" iyy="0.000283500005025417" iyz="0" izz="0.000323999993270263" />
    </inertial>
    <visual>
      <origin rpy="-4.712389 0 0" xyz="0 -0.01 0" />
      <geometry>
        <cylinder length="0.02" radius="0.025" />
      </geometry>
      <material name="black" />
    </visual>
    <collision>
      <origin rpy="-4.712389 0 0" xyz="0 -0.01 0" />
      <geometry>
        <cylinder length="0.02" radius="0.025" />
      </geometry>
    </collision>
  </link>
  <link name="front_right_wheel">
    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0" />
      <mass value="0.001" />
      <inertia ixx="0.000283500005025417" ixy="0" ixz="0" iyy="0.000283500005025417" iyz="0" izz="0.000323999993270263" />
    </inertial>
    <visual>
      <origin rpy="-4.712389 0 0" xyz="0 -0.01 0" />
      <geometry>
        <cylinder length="0.02" radius="0.025" />
      </geometry>
      <material name="black" />
    </visual>
    <collision>
      <origin rpy="-1.570796 0 0" xyz="0 -0.01 0" />
      <geometry>
        <cylinder length="0.02" radius="0.025" />
      </geometry>
    </collision>
  </link>

This is the definition for the transmissions:

<transmission name="rear_left_wheel_transmission">
  <type>transmission_interface/SimpleTransmission</type>
  <joint name="base_link_rear_left_wheel_joint">
    <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
  </joint>
  <actuator name="rear_left_wheel_actuator">
    <mechanicalReduction>1</mechanicalReduction>
    <hardwareInterface>VelocityJointInterface</hardwareInterface>
  </actuator>
</transmission>

<transmission name="rear_right_wheel_transmission">
  <type>transmission_interface/SimpleTransmission</type>
  <joint name="base_link_rear_right_wheel_joint">
    <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
  </joint>
  <actuator name="rear_right_wheel_actuator">
    <mechanicalReduction>1</mechanicalReduction>
    <hardwareInterface>VelocityJointInterface</hardwareInterface>
  </actuator>
</transmission>

<transmission name="front_left_wheel_transmission">
  <type>transmission_interface/SimpleTransmission</type>
  <joint name="base_link_front_left_wheel_joint">
    <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
  </joint> ...