As the flats were loaded, my guess would be a trip through Ipswich Tunnel to the West Bank Terminal. At that time I was working on the construction of the Orwell Bridge and every day there were small container vessels arriving at the docks from Europe.
Agreed. At the time, there was a terminal called 'Griffin Wharf' on the western banks of the Orwell; there is an account of a working from there in 'Life and Times- Freightliner'. The wagon works was next to it. A relatively recent 'Quail' map suggests the track may still be there, running from a junction with the main line at Halifax Junction, on the London side of Ipswich Tunnel. There was a second terminal on the docks line from there to Ipswich Lower Yard, but that line is shown as having been severed.