Ants need dry soil to nest in, so if you keep the rose well watered this should stop them. Ants feed on the nectar of aphids so check the rose for greenfly - could be the reason that they have visited this pot.

If they haven't set up house under the pot itself there must be a cavitation in the soil/root ball itself that they call home. Take a sharp long flexible box cutter and free the root ball from the edge of the pot and pull the plant out and you will either find nothing or a nest of ants. One half of these will be carrying their larva away from danger and the other half will be heading towards your hands to settle matters. Leave the rootball out in the sun or exposed for a few hours until the have set up camp somewhere else or give the ball a light spray of ant killer. Then place the plant back in the pot and water well.

buy the ant stop crystals. put a few around their 'run' and let them walk the chemical into the nest. We have this problem and I keep them under control this way.

A 50/50 solution of borax and sugar will also get rid of ants.

Thank you, everyone! I have started with a good soaking as suggested by Bulba and if that doesn't work I'll try the other options you have suggested. Loosestrife2, I have never heard of a box cutter but it sounds a useful tool to have as I have a number of pots where it is really difficult to take out old plants. I'll definitely try to get one.
Penny