As Ken suggests, winter damage looks to be the culprit. I have a Pinus sabiniana that looks exactly like that here in zone 5.

I'm based in Wales so Zone 8
The pines I've grown are all said to be good such as this Pinus coulteri. It was doing lovely until 2weeks after planting to the spot only 100meters away from where it has been in its pot for the past 2years.

Many thanks for your help...
Fingers crossed it'll pull through!

Cheers

Stuart, just to rule out the worst-case scenario, that of fungal infection, you might want to run y our hand lens up and down some of those browned needles, looking for tiny fruiting bodies, AKA pycnidia, to assure yourself there isn't some pathogenic process going on here. Not, BTW, saying I suspect such. But if it were me, I'd want to make certain, while there still may be an opportunity to mitigate.

What would be the best way to deal with such a thing?

Begin by observing. Just take a good close look with a magnifying lens. The fruiting structures are visible if present.