Nematodes are roundworm aren't they? Don't most/all wormers deal with them?
Nematodes is the technical term for roundworms.
Nematodirus is a specific worm, which tends to cause a problem when weather conditions are right to cause sudden hatch of multiple eggs on pasture at a time when young lambs have started grazing. It can cause scour, poor thrift and sudden death.
As mentioned above, scops will give tremendous advice on worming in general and weather conditions and how they affect the risk of nematodirus, but nematodirus risk also depends on your grazing, how clean your pasture is and your grazing rotation, as well as when you have had lambs born in relation to the risk period.
If you ask your vet, they will be aware of your local conditions and can tailor a worming plan to your specific field use, lambing and breeds of sheep so as to give best treatment and prevention while trying to reduce the speed of development of resistance in worms.