Hi Liam
It seems Peter is more liberal than me
the hackle on there looks too long to me. I'd also suggest you start the hackle too close to the eye.
I like the hackle on my dries to be shorter than your example. I use the gape of the hook to judge the length of the hackle barbs - in my example the hackle is around 1 and a third to one and a half times the gape.