There are better ground cover plants for dry soils IMHO - see other question

If this is a young plant with new growth is it being eaten by slugs and snails ?

I have used this plant in every garden i've ever had and it's a tough plant but if it's not able to 'get itself going' i suppose it could look like that in the centre.

It'll take a lot of abuse though, perhaps cut it hard back and see how it fares.

I have to say though, in my current garden i have a lot of this under hedges and trees and shrubs where it's painfully dry and it flourishes there - i'd actually say it was particularly good in dry shade.

I agree and would cut it right back to soil level. This will encourage the underground rhizomes to send out new shoots and will also give the true roots a breather and catch up. Do this in spring, though having this plant myself I know that it wouldn't make a heck of a lot of difference to it once its established as to the time of year you did this.

Thank you all for your excellent advise. We're working on it. I'm gradually removing some of the plants in preparation