Leylandii Hedge Problems
I have looked at the photos you sent regarding your leylandii hedge and have spoken to a few people concerning the cause.
Brown patches are more common on leylandii (Leyland cypress) and can be caused by various factors. Having looked at various sources of information, 30 years is very good for these trees as 10-20 years is the average age before they start to die and need removing. Looking at the photos I don’t think the patches are caused by any fungus or bugs. If you look on the RHS website it gives you a list of diseases and pests.
When the hedge is trimmed could be factor in brown patches appearing. Iif you give it an autumn trim (mainly October), even a light one, this can cause die back and cutting through the summer, when it’s hot, can stress the bushes again causing brown patches.
I have noticed over the last couple of years, with the hotter summers we are having, that conifers and leylandii are dying due to the lack of rain especially this year with the prolonged absence of water. Also when bushes are planted close to each other, like yours, there will be competition for whatever water there is available and inevitably
the weakest will die.
Unfortunately, once you have brown patches, given the age of the hedge, it is highly unlikely for those areas to ever grow again.
I hope my information is of some help.