Hi Connor,
I discovered what the problem I was having was.

in phpMyAdmin I was actually entering NULL in the field rather than checking the check box for null.

I guess the typed in chars NULL was interpreted as a string & being invalid for the field converted to 0



It all works perfectly now. It is a really great plugin.

Sorry for needlessly taking up your time.

As soon as I get paid for this job I'll be donating.

Thanks

Ps and next time I'll install the plugin before the client adds 1200 products d'oh!