You beat me to it Robert! I have just posted this on our local site, and then came here. This takes the thread a tiny bit further...
At first I was wrongly questioning whether Redherring's chassis was a leyland, as conventional hangers appeared in all pictures of the old jiggers, but I have done some scratching around;
In the Leyland Society Journal No14 Mike Sutcliffe writes; "Another survival of the practice was the cast iron dovetailed spring slides. these were quite good and gave a lot less trouble than the early shackles which often broke through 'locking' in the brackets"
It seems that all the Leyland steamers used the sliding spring fixings (fitted under RSJ rails), and that this system carried over to the first production run (class Y, 30 odd built) of petrol trucks between 1905 and 1906. The sliders were at the front only and instead of going under the rails were supported by outriggers fixed to the top of the rails. The next significant class was the X which ran from 1908 to 1912 and this used more conventional hangers fixed to the side of pressed chassis rails.
Now the mystery; The subsequent S class also used conventional hangers but there were some examples which used sliders , and the best picture I have found is in Klappers 'British Lorries 1900-1945' pp 104 with the caption "A 1914 specimen that in 1934 was still running 500 miles a week and with a trailer shifting 10 ton loads". I have not found there to be a correlation between the use of sliders and the joggled frame, as might be expected or bus or lorry use. Maybe Mike Sutcliffe will provide some answers when his history of early Leyland truck gets to the S class.
Regarding yawing the torque tube is fixed rigidly to the diff, making a solid 'T', so would be controlling both rotational torque and yawing. The springs would still play a role in controlling lateral movement.