I cut the leaves and spent flowers off mine back in September I think. You can do it now if you want, or leave them in place for added protection to the crowns, up to you.

If they are the evergreen sort of Agapanthus then they will not be hardy, so they will need winter protection. The hardy kind should have died back and can be tidied up.

Many thanks to you both, I will cut mine back now as they are going yellow and soft.

I would have left the leaves of the deciduous sorts to die back naturally, anyway, for the same reason that you would bulbs.