Keeping in mind that conditions are quite different here, I'll tell you what I know. In our area, EVERY Leyland Cypress eventually develops Seiridium Canker and dies. Here's a link to some information from our local Cooperative Extension Office:
Leyland Cypress is no longer recommended for planting here. Some sources speculate that the disease is actually transmitted at the time the plants are propagated from cuttings in the nursery trade.
I hope that this is not the case in your area!