Angular custom form validation $error not working

This is one of those bugs which has you tearing your hair out.

I have written a directive to perform a custom validation on either a time, distance or points total according to the value chosen in the select box. Here it is working nicely:

http://plnkr.co/edit/FECdILYtahR0HjoWysTr?p=preview

The problem starts when I apply the directive to my application. The form's $invalid property is set to true but the $error.mymeasure, although apparently set to true in the debugger, does not have any effect on the view.

Chrome debugger output screenshot: http://prntscr.com/2peomt

Code to output the $error and $invalid properties:

entriesForm.savedcurvalue.$error.mymeasure: {{ entriesForm.savedcurvalue.$error.mymeasure }} 
  / $invalid: {{ entriesForm.$invalid }} <br/>

Actual output in the view:

entriesForm.savedcurvalue.$error.mymeasure: / $invalid: true

As far as I can tell the relevant code is identical to the test... here's the html: (The input with the directive is at the bottom)

<form name="entriesForm" class="css-form" novalidate>
<div class="grey-border" ng-show="entriesData.currentAthleteIndex != undefined">

<!--********  ALREADY SAVED ********-->
<div ng-show="entriesData.showEventListGroup=='saved'">
    <ul class="inline entry-table-head">
        <li class="entry-col-1">{{ 'entries.class' | i18n }}</li>
        <li class="entry-col-2">{{ 'entries.event.curValue' | i18n }}</li>
        <li class="entry-col-3">{{ 'entries.event.curWeightHeight' | i18n }}</li>
        <li class="entry-col-4">{{ 'entries.event.datDate' | i18n }}</li>
        <li class="entry-col-5">{{ 'entries.event.strTown' | i18n }}</li>
        <li class="entry-col-6">{{ 'entries.event.bolindoor' | i18n }}</li>
    </ul>
    <span ng-show="entriesForm.measurement.$error.measure">{{ measurementFormat }} </span>
    entriesForm.savedcurvalue.$error.mymeasure: {{ entriesForm.savedcurvalue.$error.mymeasure }} / $invalid: {{ entriesForm.$invalid }} <br/>
    <ul class="unstyled">
        <li ng-repeat="event in currentEditItem.athleteList[entriesData.currentAthleteIndex].events"
            ng-show="event.selected && !event.isRelay">

            <ul class="inline">
                <li class="entry-col-1"
                    ng-class="{requestStatusGreen:event.ownEvents && event.ownSex, requestStatusYellow:!event.ownEvents && event.ownSex, requestStatusRed:!event.ownEvents && !event.ownSex}">
                    <label class="checkbox inline">
                        <input type="checkbox" ng-model="event.selected" ng-click="setIntStateSave(event,currentEditItem.athleteList[entriesData.currentAthleteIndex])"/>
                        {{ event.strIdClass }} - {{ event.strName }}
                    </label>
                </li>
                <li class="entry-col-2 control-group" ng-class="{error:entriesForm.saved_curValue.$error.measure}">
                    <input class="input-mini" type="text" ng-model="event.curValue" ng-change="setChecked(event)" name="savedcurvalue"
                           smart-measurement="event.intType" errorformat="measurementFormat" decimal="DECIMAL" />
                </li>

And the directive:

app.directive('smartMeasurement', function () {
    return {
        require: 'ngModel',
        scope: {
            ngModel: '=',
            smartMeasurement: '=',
            errorformat: '=',
            decimal: '='
        },
        link: function (scope, elm, attrs, ctrl) {
            var REGEX;
            var TIME_REGEXP;
            var LENGTH_REGEXP;
            var MULTI_REGEXP = /^(?:\d{1,5})?$/;

            if (scope.decimal == ",") {
                TIME_REGEXP = /^(?:\d+:)?(?:[0-5]\d:|[0-9]:)?(?:[0-5]\d|\d|^\d\d\d)(?:[.,]\d\d?)?$/;
                LENGTH_REGEXP = /^(?:\d{1,3})(?:[,]\d)(\d)?$/;
            } else {
                TIME_REGEXP = /^(?:\d+:)?(?:[0-5]\d:|[0-9]:)?(?:[0-5]\d|\d|^\d\d\d)(?:[.,]\d\d?)?$/;
                LENGTH_REGEXP = /^(?:\d{1,3})(?:[.]\d)(\d)?$/;
            }

            ctrl.$parsers.unshift(function (viewValue) {
                switch (scope.smartMeasurement) {
                    case 3:
                        REGEX = LENGTH_REGEXP;
                        scope.errorformat = "00" + scope.decimal + "00";
                        break;
                    case 4:
                        REGEX = MULTI_REGEXP;
                        scope.errorformat = "00000";
                        break;
                    default:
                        REGEX = TIME_REGEXP;
                        scope.errorformat = "[hh:mm:]ss" + scope.decimal + "t[h] / [s]ss" + scope.decimal + "t[h]";
                        break;
                }
                if (REGEX.test(viewValue)) {
                    ctrl.$setValidity('mymeasure', true);
                    return viewValue;
                } else {
                    ctrl.$setValidity('mymeasure', false);
                    return undefined;
                }
            });
        }
    };
});

I am using Angular 1.06 - The project was started almost a year ago so upgrading to 1.2 is not really an option at the moment. I am thinking that perhaps one on the dependencies could prevent it working. I will test this although I find it unlikely. But that is the only thing I can think of that might be wrong.

If anybody can sport an error or suggest a place I should look, it would be very much appreciated.