This is potentially very difficult and very expensive.
Trying to dig and pour a footing down there will be a nightmare/ read expensive and it has the very real possibility of being undermined by water during storm periods filling the gulley.
The best bet if you want to go all out would probably be gabions.
Where is your boundary? IF its in line with the trench you dug then even though you may loose 6 or 8 inches i would just try to re-instate the fence and live with the compromise. You could easily spend mega bucks on this and still not get a worthwhile permenant solution.
Dig holes as deep as you can (600mm at least) and put in concrete posts.
Is that just a garage or you house in the side of pic 2? I'm very surprised it was built so close to the slope without some sort of retaining structure in the slope. Or maybe it was piled or on a heavy raft footing
is the onus on you to sort this out? Any chance of the field owner contributing?
Gabions are good because they will handle water better. Re-inforced concrete retaining walls like that need big wide foundations.
For example i worked on one in a similiar scenario that had to be hand dug. It was a 4m high wall and the footing was 1600mm wide with plenty of steel. It was grim. Thats before you;ve started shifting and laying blocks.
The big consideration here though would be the possible undermining of a footing by the water course. Is there any actual flow through it or is it usually a dry ditch.
Personally i wouldn't risk a full on masonary wall. It is just more at risk of failing. As i said you could spend a decent chunk on it and it still goes wrong a few years down the line.
If it was me i would spend the equivilant on gabions. Excavate the same way forming a nice wide base cut well into the slope and then fill them. At least they will tolerate any movement whereas a wall will just fail.
They will probably be similar money due to the extra excavation needed.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local