Is my hedge dying??

That doesn’t look too bad @martinlest. I’d follow the advice above to try and encourage other neighbouring branches to grow over the gap (and hide those butterflies asap .) Give it time and see how it progresses.

 

Thuja leaves when crushed have quite a nice smell, it’s described as like pineapple but I’m not sure I would describe it as that.

 

I guess it could be worse, but the brown 'crumbliness' is just below the surface, all over the hedge. Maybe that's just how that species grows (??).

 

I agree- that looks ok. The worst bit IMO, is to the left of the butterflies, near the base. That will be very difficult to improve, and it often happens because the top is left to grow wider than the base, rather than the other way round, and therefore shades out the base of the hedge, but you can always have a couple of containers there to hide it, or some low growing planting.
It should be easy enough to train a few bits across the top bits though, and wire them in.
It actually looks like two different varieties though. The bits on the left look much lighter then the right hand side, but it might just be the pic.

Just one thing - that looks like an ash tree sapling, just to the right of the table. If so, I'd get that out now before it gets any bigger!

 

Yes, it is. Thank you for the heads up!! I'll dig it out while I still can.

And you are right of course about there being two varieties: the more yellow, on the left, is the browner of the two, with most of the foliage dead and crumbly an inch or two in from the exterior.

 

There's a gold version of leylandii as well as the green variety. They both act the same way, as far as I know, although the gold seems to be less common for some reason. I've never had it in any garden so I don't know if it's more suspectibe to weather/general conditions etc.

You could take a look at some of the hedging suppliers to see what they say about it though.
I've just mentioned this one - The UK's Leading Garden Hedging & Hedging Specialists | Hopes Grove Nurseries on another thread as I've used them a few times, but there are several good hedging suppliers - https://www.best4hedging.co.uk/ and https://www.hedgesdirect.co.uk/ are another couple thatpeople seem to find good.

 

I'll take a look. Thank you.