Those leaves look healthy enough Hollins but has your plant ever flowered? The flowers are tall and very attractive but the plant looks silly after I have cut all the mouldy leaves off it and I have to make sure that my dog doesn't walk on them as they are so spiky.
I don't know much about the plant apart from what Nemesis has said so I looked up on a Q & A on the plants
Q Why doesn't my Acanthus flower?
A This is probably due to there being snails in the crown of the plant. The flowers spikes are lovely and soft and extremely attractive to molluscs. Snails are often seen “sliming” along the leaves after it has rained. Another reason can be that the plant has been caught by a very sharp frost and the flower buds have been spoilt. The last cause could be exacerbated by the soil being too rich or if the plant is fed with too much nitrogen. In this instance, the plant is too well fed to need to flower or set seed.