Commerce 2 Adding tax to custom adjuster
Having slept on it i thought it could be something to do with the order of the adjusters. Maybe Tax was applying to the price before my custom adjuster. So in the event i did the following:

var_dump($event-&gt;types)


This told me that my custom adjuster was going to the bottom of this array. To resolve this i used the php function array_unshift to push it to the front of the array

array_unshift($event-&gt;types,MyAdjuster::class);


This now gave me the following array:

array(4) {
      [0]=&gt;
      string(18) "modules\MyAdjuster"
      [1]=&gt;
      string(33) "craft\commerce\adjusters\Shipping"
      [2]=&gt;
      string(33) "craft\commerce\adjusters\Discount"
      [3]=&gt;
      string(28) "craft\commerce\adjusters\Tax"
    }


As you can see the adjuster is at the front of the array and now the tax applies to the fee as required.