If the foliage is browing from the centre of the tree, scattered and working its way out along the branches it is very suggestive of a soil borne fungal infection such as Phytophthora root rot to which conifers are particularly susceptible to. This type of root rot occurs most frequently in warm wet soils. Given that you have watered it regularly and it is planted in a sheltered position, I would imagine that such conditions are present. There are no chemical cures available for this problem so it is recommended that infected plants are removed and destroyed.
Prevention is generally better than cure. If you decide to replace the plant you will need to improve the soil well before replanting and avoid excessive watering and mulching with manures that hold water in the soil. If you are gardening on heavy clay then perforate the sides of the planting hole before planting your tree to ensure good drainage.