I have added the HP Product Number as per you instructions (Admin, Feed Custom Data)

Should the Warranty dates now update Automatically or do I have to manually go to each machine and hit scan for the Warranty fields to update?

 

It clearly states when doing the above:

The existing Product Number, Shipping Date and Expiry Date will be overwritten with the above mentioned values, which will also disable automatic warranty scan.

 

As you have to key the HP Product code manually surely it doesn't turn off the Automatic warranty scan!