Darren,
I checked out your site and here is the button code you have for your PVC 3 Piece Set product:
form action="
https://www.e-junkie.com/ecom/gb.php?c=cart&i=350932&cl=87018&ejc=2" target="ej_ejc" method="POST" accept-charset="UTF-8">
<input type="hidden" name="on0" value="Size"/>
Size:<br/>
<select name="os0">
<option value="32" Bust">32" Bust</option>
<option value="34" Bust">34" Bust</option>
<option value="36" Bust">36" Bust</option>
<option value="38" Bust">38" Bust</option>
<option value="40" Bust">40" Bust</option>
<option value="42" Bust">42" Bust</option>
</select>
<br/>
<input type="image" src="
http://www.e-junkie.com/ej/ej_add_to_cart.gif" border="0" alt="Add to Cart" class="ec_ejc_thkbx" onClick="javascript:return EJEJC_lc(this.parentNode);"/>
</form>
I then checked out your account in our system and here is the button code directly from your E-junkie account. Notice there is a slight difference in the code for the bust size. Try placing this code on your page to see if that fixes the issue:
<form action="
https://www.e-junkie.com/ecom/gb.php?c=cart&i=350932&cl=87018&ejc=2" target="ej_ejc" method="POST" accept-charset="UTF-8">
<input type="hidden" name="on0" value="Size"/>
Size:<br/>
<select name="os0">
<option value="32" Bust">32" Bust</option>
<option value="34" Bust">34" Bust</option>
<option value="36" Bust">36" Bust</option>
<option value="38" Bust">38" Bust</option>
<option value="40" Bust">40" Bust</option>
<option value="42" Bust">42" Bust</option>
</select>
<br/>
<input type="image" src="
http://www.e-junkie.com/ej/ej_add_to_cart.gif" border="0" alt="Add to Cart" class="ec_ejc_thkbx" onClick="javascript:return EJEJC_lc(this.parentNode);"/>
</form>