the select list is passing text values:
<select name="LEDCart_1_Dimmable_Add" id="LEDCart_1_Dimmable_Add">
<option value="Non-Dimmable">Non-Dimmable + £0.00 </option>
<option value="Dimmable">Dimmable + £0.70 </option>
</select>
it should be passig the option ID to be used i nthe lookup recordset.
the lookup recordset is set to filter the LEDprodoptions.LEDprodoptionsID column on the value passed by the LEDCart_1_Dimmable_Add select list.
since you are passing text values, the lookup on a numeric column is failing.