I have a set of rods which I use - the old drains have (i think) bits of roots that catch stuff. Bit worried the jet wash would just spray shit everywhere, but it is probably worth a go when there isn't a pool of stagnant shit to get it properly clear.
Making me gag again thinking about it.
I've done it once, using a pressure washer & jet hose attachment.
Never again. I will gladly pay the ~£100 it costs to have someone else come and do it.