If you view source of your page, the LocIDform element is in a separate form from add to cart button.
<td class="info"><form action="month.php" method="post" name="loc209" id="loc209">
<input name="LocID" type="hidden" value="2" />2. </form></td>
<td class="info">16 x 24 in</td>
<td class="price">£75.00</td>
<td class="form"><form action="/test/month.php?Month=1" method="post" name="L365cart_1_ATC_" id="L365cart_1_ATC_">
<input type="hidden" name="L365cart_1_ID_Add" value="209" />
<input type="hidden" name="L365cart_1_Quantity_Add" value="1" size="4" />
<input type="image" src="images/design/buy.gif" border="0" value="Add to Cart" name="L365cart_1_ATC" alt="Buy now!">
</form></td>
Move the LocIDform element so that it is part of the Add to cart form.