Thanks again Jason.
The snippet you've given was giving me the white screen so I've had a look in eCart1_PHP.php and found the two place where that is inserted and have manually edited them to show this:
if($itmObj->Quantity > $itmObj->StockLevel) {
$itmObj->Quantity = $itmObj->StockLevel;
}
(my column for stock is StockLevel)
and this is now working. However, it is screwing up the Cart Object dialogue so I would be keen to do it properly. Are you able to suggest what the problem could be?
Thanks again
Jon