I'm not 100% certain on this but there are reports on here for Trowse Pumping station and information at
https://www.geograph.org.uk/photo/1672752 seems to suggest that you have explored the 1909 extension to the building, despite it being called The Granary. The mill at Trowse burnt down
http://www.norfolkmills.co.uk/Watermills/trowse.html and so the equipment you saw may be more connected to the pumping station than a mill. Lack of further details prevents me being 100% sure, but if you do go back see if you can find the foundation stone from 1909