i have a 1972 sprite [virtually same as nimbus] and when i got it about 15 years ago like all old dumpers it had a huge amount of play in the steering box.
in 2007 i managed to get some replacement parts for the steering box [shims and the like] and most importantly a service manual and parts list and diagrams from a company called leach lewis plant nr sevenoaks , contact simon ede on 01732 78155. they are the thwaites distibutor for the south east. there are lots of otghers.
this improved the steering quite a bit but still not satisfactory.
i continued keeping a saved search on ebay for 'dumper steering' and in due course in 2016 i saw an advert for a brand new 'steering box and column, dumper milk float digger tractor' for £185 being sold by thwaites. i got in touch with glenn at thwaites allparts 01926 466016. it transpired that they had been clearing some old sheds and had found some old but unused spare parts. i established that it was not exactly the same [ it is a
marles P3688A and the original was i think a marles P3890 ] but in conversation with glenn i thought i could make it fit. the attraction of being unused was the decider. i bought it and the drop arm [ £75 extra ].
the drop arm had different taper directions so i cut it in half and welded it to match my drag link. the steering box clamp [a big heavy casting] was too small so i had it turned out to fit [ cost me £60] and bobs your uncle i've got brand new steering.
some other names ;
david cornwallis leominster rebuilds thwaites dumpers [google]
john lewis dumpers bromyard 01544 260625
i think if you google and use websites like this you will find something.
i was very lucky, but i had not restricted my search to thwaites parts or even marles boxes. these heavy duty steering boxes were and no doubt are still used on all sorts of vehicles.
best of luck