Are you going to plant the clematis in a pot? It helps to put pots on feet - that way the ants have a harder time getting in.

There's no point adding nematodes to soil on the off chance that some ants might appear later. The nematodes parasitise the ants if they meet them. If there are no ants present the nematodes will die. That's just a waste of your money buying them.

Also, there is quite a narrow temperature range in which you can add a nematode suspension to soil and it's almost certainly too cold now to use them.

Ants hibernate, so you will see less of them over the next few weeks. I always thought Ants are not a problem in the garden, unsightly en-masse, but not something that actually harms the garden.

Anyway, I have heard good stories of people using coffee grinds (the stuff that goes into machines not instant coffee - can ask for this from a coffee shop) Just place around the nest and garden and they leave the place alone as well as you fertilising the garden.

Plants in pots are vulnerable to ant invasion, particularly if they're on paving. As Beattie says, raise the pots just above the level of the paving by using pot feet or broken roof tiles, anything to give a bit of clearance beneath. Turn the pots out and dispose of the compost and ants, clean the pots and refill with fresh compost, preferably placing a jeycloth or similar in the bottom over the drainage holes first, not to block them, but to dissuade ants from entering via the holes. Dust the area beneath the pot with ant powder. Whilst the ants don't eat your plants, they will gnaw through roots and anything in their way if they build a nest inside a pot.

If you intend to plant the clematis in the garden then the ants will not be a problem and the clematis will thrive better than in a pot as they love moist soil and cool roots. If you must plant in a container then as suggested keeping the soil moist will also deter ants and raise the pots on feet and I have found that 'Antstop' does the trick (use in the spring). Sprinkle it in their path and it contains microscopic particles which collect on the ants' feet, they take it back to the nest and it breaks down slowly and poisions the colony. Give it a go.