Hi Ajeh,
Sorry for delay the way I worked it out was simple really I went to Developers Tool Kit and entered lb(s) and it came up with this file.

/public_html/includes/languages/english/modules/shipping/zones.php

Line #25 : define('MODULE_SHIPPING_ZONES_TEXT_UNITS', 'lb(s)');

Hopefully this will help others and I realise now that you must put the exact spelling into Tool Kit.
At first I was just putting in lbs and not lb(s) does make a difference.

Regards andrew