Apologies Ray... I will in future start a new threading this situation.
I now realise the form was completely unnecessary.
I had tried using the calculation directly.. Which would have worked but I have noticed what was wrong with my update behaviour. the filter was referring to the wrong session ID.
$UpdateQuery->addFilter("enq_det_id", "=", "i", "".$_SESSION['enquiryDetID'] ."");
Should have been
$UpdateQuery->addFilter("enq_det_id", "=", "i", "".$_SESSION['enquiriesDetID'] ."");
Working fine now.. Many thanks