I think you could have a problem trying to keep in a pot as this one can grow to a metre in height and width and is fully hardy. Why not plant in the ground?

If you can't though, just pot it into a lager one - tough old things, move it on now, it won't hurt it. But it does need a much bigger pot than that

I've had a felix mas in a 17" bell pot for years, never repotted or fed and it still comes up smiling in its dark little corner every year. Doesn't reach the full height it could in the ground but its fine - and affinis doesn't get as tall as felix mas.

Thank you MG for your answer I understand like most plants they prefer being in the soil. However, the place I have in mind for this fern means that I am pretty much confined to having it in a container. In this case I will go with steragram's answer and find the biggest pot I have and plant it in that and hope for the best, it would appear that no harm has befallen your plant over the years. Many thanks to both of you for replying to my question.~ phloxie.