SFX we have same problem here, but Irish nits
we used Hedrin for three weeks running and it hasn't worked at all
Best and cheapest treatment is coat wet hair with any conditioner and comb through with lice comb. Conditioner slime means they can't crawl away as it paralyses them and it helps make the eggs slide off. You still need to do it once or twice a week for three weeks but it means you can avoid the nasty chemicals
Head lice are immune to most of those expensive "treatments" anyway unfortunately
In between treatments, spray on tea tree oil spray to deter new lice from setting up a colony on DDs head
there are lots of gentle herbal lice and dandruff treatments about too
I have itchy scalp and find Occitane shamppoo is good as it doesn't irritate it