I'm far from a Ditch-Master but the one thing that I think could have saved your fish in
your situation was like Mike said,..."
climb and walk", (while keeping tension on the line of course
)
Sometimes even with the best tackle,, you have to be quick on your feet when it's a big fish swimming with the current. I know it's easier said than done....tough spot to climb-up, other fishermen in the way, only having 1 leg etc....but you always have to be ready for that situation when fishing the ditch. If somebody hooks a 50 in the ditch, they better walk the fish no matter what they're using IMO.