Often it is the case that the mitre joint on a cill opens up. This happens a result of shrinkage usually after a prolonged spell of dry weather. The joint will remain open and this will allow moisture to penetrate over the winter months. Wet rot will follow.
A simple cost effective remedy is to insert Oak Slips. In this case 3.5mm thickness, Oak being a superb weather resistant material will remain in place without rotting for many years to come. By doing this the mitre joint always a weak spot will remain intact preventing wet rot to get a hold. 
Carpentry Services Southampton 