Apart from the obvious (that it is slowly dying!)
Any thoughts on what could be causing this?
I have used a weed spray in the area, but have been careful to keep it at least 1-1.5 mtrs away from the hedge.
Hi Justin
I cant really see from the photo, it looks like a single branch dying or is it the whole plant? If it is a single branch I would be pruning it off immediately so that the disease most likely fungal doesn't infect the rest of the plant. Cypress Dieback appears to becoming more common this year due to many dry years followed by and extremely wet summer. Always disinfect pruning tools before, during and after pruning infected plants.