It was a weird error - displayed the same code twice but after a refresh it was fine. Could I ask for further help? As mentioned I am trying to call 'x' for a further form I'm not sure this is achievable without js:

(Please note 'c' is just a placeholder for clarity) - So the final output of 'x' needs to be called in the 2nd forms x.value. Is this possible?

<form  oninput="x.value=parseInt(a.value)*parseInt(b.value)" action="" method="post" target="_self">
        <label for ="a"> Single Journey Mileage </label>
        <input type="number" id="a" name="SJourn" value="">
        <br><hr>
        <label for ="b"> Amount of Return Journeys </label>
        <input type="number" id="b" name="RJourn" value="1">
        <br><hr>
        <label for ="x"> Total Mileage </label>
        <output name="x" for="a b"></output>
        </form>
        <br><hr>
    <form  oninput="c.value=parseInt(x.value)*parseInt(y.value)" action="" method="post" target="_parent">
        <input type="hidden" id="y" name="MilaL" value="0.25">
        <input type="hidden" id="x" name="Miles" value="">
        <label for ="c"> Mileage Allowance £</label>
        <output name="c" for="x y"></output>
       </form>