OS Explorer Map 197 coordinates 3038 for Deben Lodge Farm and the start of the track, neatly marked out with green diamonds (on the map, not for real).
I went out looking for this track twice before I found it. That followed a ride into Felixstowe when I found myself on the old dual carriageway near Levington being overtaken by cars travelling at scary speeds. I felt sure there must be a way to get to Felixstowe from Falkenham and eventually I found it.
From Kirton follow the signs to Falkenham. Ride through Falkenham and turn left down the road that says ‘Deben Lodge farm only’. There’s more than just a farm down there and you will find a gravelly track that runs alongside the Kingsfleet tributary. When you emerge at the far end of the track you get to Felixstowe Ferry via Marsh Lane and Ferry Road, through the golf course.
Have a cuppa at the cafe and watch the Bawdsey Ferry do its stuff. £3.50 one way with a bike will get you to the other side and a ride up towards Woodbridge. Alternatively save the ferry for another day, turn around and go back the way you came. Either way its around 30 miles altogether.