The following advice on the treatment of head lice from NHS choices website:
Head lice can be difficult to treat due to a high re-infestation rate and their ability to develop resistance to traditional insecticides contained in some medications.
It is thought head lice will not develop immunity to the newer silicone and oil-based preparations because they have a physical rather than a chemical action on lice.
After a head lice infestation has been confirmed, you can treat the lice at home by wet combing the hair using a head lice comb or by using medicated lotion (see below).
However, neither will protect against re-infestation if head-to-head contact is made with someone with head lice during the treatment period.
WET COMBING METHOD:
The wet combing method involves removing the head lice by systematically combing the hair using a special fine-toothed comb. The comb’s teeth should be spaced less than 0.3mm, but at least 0.2mm, apart. Lice can be trapped between the teeth of nit combs with a tooth spacing of less than 0.19mm and remain unseen. Combs are available from pharmacies. The use of medicated products is not necessary for wet combing. This is advantageous because head lice are becoming more resistant to the insecticides commonly used to remove them.
However, for wet combing to be effective, it involves regular and thorough combing which is time consuming. The wet combing method is described below. Wash the hair using ordinary shampoo and apply plenty of conditioner, before using a wide-toothed comb to straighten and untangle the hair.
Once the comb moves freely through the hair without dragging, switch to the louse detection comb. Make sure the teeth of the comb slot into the hair at the roots with the bevel-edge of the teeth lightly touching the scalp. Draw the comb down to the ends of the hair with every stroke and check the comb for lice.
Remove lice by wiping or rinsing the comb. Work methodically through the hair, section by section, so that the whole head of hair is combed through. Rinse out conditioner and repeat the combing procedure in the wet hair. Repeat the procedure on days five, nine and 13 so that you clear young lice as they hatch, before they have time to reach maturity.
The length of time it will take to comb your child’s hair will depend on the type of hair your child has and how long it is. For example, short, straight hair can be quickly prepared and can be fine-toothed combed in a few minutes, whereas longer, curlier hair will take longer to comb.