Everyone and anyone who knows me, knows that I am a big supporter of and I champion the cause of ground driving and long line work. Fix the horse first and we can work on fixing us later.
For the English crowd long line work is not lost if you don't have a surcingle. Use what you have- saddle, bridle and a piece of baling twine.
Drop your irons and use the baling twine to tie them together underneath the horse. This is a little closer look and you can see the blue baling twine on the inside of the iron. Just like with a western saddle, this will limit the swing and keep them closer to the horse more like a surcingle.
To keep your reins up and out of the way you can run them under your leathers and buckle them on top of the seat. This will still allow your horse plenty of room to stretch out and down when working.
In the past I have also seen the reins run under the flap and buckled on top of the seat. This shortens them a bit and works more like side reins, but for long line work- this doesn't help. You don't want your reins giving one signal and your long lines another. You can also either take them off altogether or twist them up, buckled around the neck and run thru the throatlatch on the bridle. Whatever works for you. Now you're ready to get to work and fix the horse.