Amazon MWS SubmitFeed Updating stock quantity

I'm having problems with a stock quantity update feed using Amazon MWS. My Feed is submitted and processed, but I get errors, however if I submit the same XML via the scratchpad, the inventory updates are accepted and processed.

(merchant id starred out deliberately)

Submission and response below:

<AmazonEnvelope xmlns:xsi="http://ift.tt/ra1lAU" xsi:noNamespaceSchemaLocation="amzn-envelope.xsd">
<Header>
    <DocumentVersion>1.01</DocumentVersion>
    <MerchantIdentifier>************</MerchantIdentifier>
</Header>
<MessageType>Inventory</MessageType>
<Message>
    <MessageID>1</MessageID>
    <OperationType>Update</OperationType>
    <Inventory>
        <SKU>BUS999904</SKU>
        <Quantity>269</Quantity>
    </Inventory>
</Message>
<Message>
    <MessageID>2</MessageID>
    <OperationType>Update</OperationType>
    <Inventory>
        <SKU>PROBS-HO-01</SKU>
        <Quantity>137</Quantity>
    </Inventory>
</Message>

And the response:

<?xml version="1.0" encoding="UTF-8"?>
<AmazonEnvelope xmlns:xsi="http://ift.tt/ra1lAU" xsi:noNamespaceSchemaLocation="amzn-envelope.xsd">
<Header>
    <DocumentVersion>1.02</DocumentVersion>
    <MerchantIdentifier>M_ONTRACKSCO_1106147</MerchantIdentifier>
</Header>
<MessageType>ProcessingReport</MessageType>
<Message>
    <MessageID>1</MessageID>
    <ProcessingReport>
        <DocumentTransactionID>54774016520</DocumentTransactionID>
        <StatusCode>Complete</StatusCode>
        <ProcessingSummary>
            <MessagesProcessed>2</MessagesProcessed>
            <MessagesSuccessful>0</MessagesSuccessful>
            <MessagesWithError>2</MessagesWithError>
            <MessagesWithWarning>0</MessagesWithWarning>
        </ProcessingSummary>
        <Result>
            <MessageID>1</MessageID>
            <ResultCode>Error</ResultCode>
            <ResultMessageCode>25</ResultMessageCode>
            <ResultDescription>We are unable to process the XML feed because one or more items are invalid. Please re-submit the feed.  </ResultDescription>
        </Result>
        <Result>
            <MessageID>2</MessageID>
            <ResultCode>Error</ResultCode>
            <ResultMessageCode>25</ResultMessageCode>
            <ResultDescription>We are unable to process the XML feed because one or more items are invalid. Please re-submit the feed.</ResultDescription>
        </Result>
    </ProcessingReport>
</Message>

Any help anyone can give, or pointers/examples of valid stock update feeds would be most welcome.

Thanks.