just get some ant killer powder that you get from most garden centres .

The right chemical, but probably the wrong form. As Np says, the same active ingredient in powder form is much more effective. Scatter the powder on the soil surface, right after a watering. The organic method would be to wrap a band of masking tape around the base of the trunk, and squeeze a thick bead of petroleum jelly onto the tape. Wipe off and replace the jelly as it becomes clogged with ants. Eventually, the hive will die, or move elsewhere.

First, as this is a houseplant, I'm wondering quite how ants have managed to get to it at all. That said, there will be a reason why they're all over the plant, assuming they're on the leaves and stems, even if you can't see any aphid infestation. The milky sap is only apparent when the leaves are damaged, or the stems are cut, and the ants will not be able to access the sap without some kind of damage to the plant occurring in the first place. It's also extremely unlikely they're after the sap anyway.
Suggest you check the stems and backs of leaves for deposits of any kind, including scale. As there's an ant problem, I'm assuming the plant is either in a greenhouse or a conservatory rather than the house, and that it may have been stood outside at some point. If the latter is the case, check the rootball by turning the plant out of its pot to make sure the ants have not built a nest inside it.

Thank you for all comments. The pot is in what we call our garden room right by the outside door. The soil had dried probably because Ive been careful with water! We definitely have a nest inside the pot. We put ant powder around the base and floor last night but the ants are still everywhere. There is a powdery deposit on some leaves, I will take it to our garden nursery for advice and possibly buy an ant trap. Think we will have to treat the root ball somehow which I was trying to avoid.

It's not difficult to do - decant the plant from the pot into a large container full of water. Leave it sitting in that for a couple of hours or so - ants hate water, don't like wet soil, so they should, hopefully vacate their nest if there is one. Once you're sure they're all dead and gone, remove the plant, empty the container of water and dead ants, refill with water and soak the plant again for another half an hour or so. Then shoosh it up and down in the water to get off some or most of the compost, and repot in fresh compost, this time with half a jeycloth laid across the bottom (or a whole one if its a large pot) to make entry into the pot more difficult. When repotted, stand the pot either in an outer container with no holes in it, or raise it off the ground with pot feet or something to give a gap of a few millimetres between the pot and the floor. Dust the space beneath occasionally with ant powder.
Powdery deposit, if its white, might be mildew, spray with 1 part milk to 9 parts water. If its black, that's sooty mould, and there'll either be scale or aphid infestation, so spray for those too, but not at the same time as the milk spray.
Note that Ficus do not like overly dry soil at the roots.

Oh thank you so much, that is so helpful. We did try washing the ants out whilst everything was intact in the pot (outside) but we will try decanting, washing and repotting. Also I think the powder could be mildew as it was in one area only where the leaves were pushed against the wall due to lack of space so there was probably no circulation of air. I love my Ficus and dont want to lose it! I must give Reggie more water from now on! I will send an update soon.

The thing they hate more than anything is draughts, followed closely by getting way too dry. Although ants in the rootball won't be good...