Your 'Twister' agapanthus is a beauty! In New York, we would need to take this plant indoors to a cool space to protect it over the winter but your agapanthus is fairly hardy in most parts of Europe. In Portsmouth, you are right on the cusp of its hardiness range so you will need to take some winter precautions.

The over-wintering procedure will depend upon whether you have 'Twister' planted in the ground or in a container. In the ground you will need to wait for the ground to begin to cool and the plant to begin to die back. Then tidy the plant and leave the location well mulched. Check over the season to ensure that the mulch hasn't heaved or drifted and replace as necessary. If in a container, some horticultural fleece can be used to insulate the entire plant and container once it has died back.

More detailed instructions can be found on this RHS fact sheet.

Courtesy of NYBG Plant Information