Mills and Millers of Hampshire volume 3 reports that there were 3 mills in close proximity at Sherborne St John on the Bow Brook.  (Actually, the stream is called Wey Brook, and joins Bow Brook at Beaurepaire Mill further north.)

It specifically mentions Matthews Mill, “possibly on the site now occupied by a water treatment works”, and gives its location as SU 626 558.  This location seems implausible being only 100m west of Sherborne Mill (circled in red).  As can be seen from the map below, the sewage works are 3-400m north of Sherborne Mill at SU 628 562.  Looking at the older map, the water works will be just below where the ‘ST’ is, but no mill is marked there. 

However, there is a very small mill pond (compared with that at Sherborne Mill) marked 200m further north still, at SU 628 564 indicated by the red star on both maps.  At that site, the stream bed can be seen to widen a bit and is filled with vegetation, and there is a bridge over the stream, but no other works are visible.

As for the third possible mill, there is no evidence on the older map, except perhaps for what could be a building on the stream to the west marked by the green star.