As Michael and HJP say, your sprocket has probably worn through the anodised coat and is now eating the soft aluminium under it. I don't think that any amount of alignment or tension adjustments will fix it. The only cure is either a steel or a Supermax poly remanufactured one.
The original alloy ones can wear out in less than 3000 miles. If you are still in warranty you may have some luck convincing your Morgan dealer that this isn't what is expected of a fairly expensive car these days and get a replacement steel one. Even if you are out of warranty it would be worth a good moan at them and see what sort of conscience they may have. It is a well known fault, you are not the first that has had this happen.
