Surely it's too early for slugs.....
Many thanks for your recent enquiry regarding the Nemaslug product we supply. These nematodes, phasmarhabditis hermaphrodita, should only be applied when the soil temperature is in excess of 5 degrees C – typically towards the middle to end of March in the Midlands, and slightly earlier and later in the south and north respectively.
After the initial application, further treatments are required every 6 weeks throughout the growing season or as required and the nematodes can be applied right up until the temperature of the soil dips down below 5 degrees again in autumn/early winter.
With regard to your other queries, it is possible to apply nemaslug to the lawn, but as slugs are not normally a pest in this environment, it is not often practised. Watering the nematodes on can be quite difficult and frustrating if you are using a watering can with a fine rose (I speak from experience in our own Harrod Horticultural Kitchen Garden!) but if you manage to find a rose with larger holes (or even adapt a plastic rose with a nail or similar object) the application process is very simple. Thoroughly stirring the nematodes when mixing with water also helps when watering onto the soil. And a further tip; if you are finding slugs in one location (in this case near compost bags), it is worthwhile collecting and disposing of them now. Although this may seem like a thoroughly unpleasant task on a cold January evening, you will reap the benefits come spring and summer when the slug population is depleted!