I have a fifeteen year old agapanthus which flowers reasonably well but looks as if it could benefit from dividing I am unsure how to go about this without harming the plant any help and advice would be very welcome
You can either divide now (do not delay it) or in early spring. It is not unlike dividing prímulas, you need a sharp, stout knife or a meat cleaver (if you have one). Remove the dead flower stalks and cut the leaves in half (this is debatable). Depending on the size of the clump, you could divide in three or four equal pieces, retaining roots to each part. Place the pieces in individual pots with good compost, water them and keep them frost-free and not wet till spring.
I've found that you may not get flowers next year, they like to be pot bound to flower.