Html.ValidationMessageFor not displaying at all

I have the following form:

<form action="~/buy-online" method="get" class="buy-online-form clearfix" autocomplete="off" id="buy-online-search">
    <div class="infield-holder clearfix">
        @Html.LabelFor(m => m.CustomerPostcode, new { @class = "infield" })
        @Html.TextBoxFor(m => m.CustomerPostcode, new { @class = "textbox" })
        <input type="submit" value="Buy Online" id="find-retailer-button" class="button" />
    </div>
</form>

@Html.ValidationMessageFor(m => m.CustomerPostcode)

Which works fine and will display an error message when submitted without jQuery, but when I add the jQuery validate scripts (v 1.11.1):

<script src="/scripts/jquery.validate.js"></script>
<script src="/scripts/jquery.validate.unobtrusive.js"></script>

It stops the form submitting but doesn't display the error message

My property is marked like so:

[DisplayName("Enter your full postcode")]
[Required(ErrorMessage = "Please enter your full postcode")]
public string CustomerPostcode { get; set; }

And the html renders like this:

<input class="textbox" data-val="true" data-val-required="Please enter your full postcode" id="CustomerPostcode" name="CustomerPostcode" type="text" value="" />

<span class="field-validation-valid" data-valmsg-for="CustomerPostcode" data-valmsg-replace="true"></span>

If I inspect the input when I hit submit it is adding the class input-validation-error to the textbox but just not updating the validation message.

All the posts that I have checked on this problem just say to include the scripts so I'm at a loss as to why the message is not showing.

I've even tried adding the jquery.unobtrusive-ajax.js script but that didn't seem to do anything either. Any help would be appreciated, thanks.