Thanks for the replies.

Had the tube replaced and it turned out to be a compression puncture. Two big holes in either side of the tube. So big in fact that even with a compressor the tube would hold no air.

Funny thing is I check pressures so religiously and I checked it before I went out that day. I didn't hit a bump or pot hole. Tube was only 100 miles old. Heavy duty one too. When it went while I was riding it went flat so fast. Scary.

Maybe the tube was pinched when it was last changed. Who knows.

Had big punctures on tubeless tyres before and they have never gone down this fast. Wish there was a tubeless option for the XT.