These insects always come from the ground upwards, usually from skirting boards or boilers. The best way to try to control ants is to use an aerosol spray, not too pungent an odour as ants are very susceptible to smells. When this occurs the ants will move any eggs that have been laid as soon as spraying starts.
Spray lightly over the areas where the ants are appearing, but not near any naked flame, i.e. backs of gas fires. Do not be tempted to overdose with the spray, the ants will deliberately not come into contact with the insecticide. A light mist will be adequate to kill the insects. Repeat this process every five to seven days for however long it takes to control the insects.
You will only ever be able to keep these insects under control. It is notoriously difficult to eradicate ants, even professional pest control companies cannot guarantee removing the problem completely. If the insects are in the kitchen, make sure all spilled foodstuffs are completely cleaned away, especially sugary substances.
The products on the market at the moment are all suitable for the job, and can be bought from most supermarkets, garden centres and hardware stores. They are not too expensive and are easily obtainable.
The instructions and precautions on the container must be strictly followed and dispose safely of the container after use. Only your persistence in spraying regularly will hopefully help to eradicate them in the future.
Finally, do not put any insecticide down for ants outside. This will cause a barrier to any ants in the cavity wall and drive them into the house.
Ants outside should be left outside where they will do no harm.
See the Pests home page.