I have done mine for the last two years in July-August, after the flowers are finished. Since our seasons are probably slightly out of kilter with the UK's, I'd suggest any time up to the middle of September, taking the new, strongly growing non-flowering shoots, which should give them time to root properly before the winter descends.

Take 2-3" semi ripe cuttings from End of july to end september. Plant cuttings in a sandy soil and give plenty of humidity and bottom heat if you can.