. . . even the OR Engineer said your not the first customer I've had using Vodafone with VDSL dropout complaints.
Do I ring Vodafone support to find out what the Engineer put on the notes to ISP or will they call me back to what was or not found ?
I would be inclined to contact VF, to update them on the result of the Openreach technician's visit. Then the above comment, about other VF customers having dropout problems, could also be relayed back.
The next item to discuss is where in the VF domain does this problem originate, as the metallic pathway has now been ruled out? A very simple test would be to substitute a known, reliable modem/router for the HHG2500. But you will need the login credentials so as to establish a working connection . . .