That looks like what's known as a 'shake' and is unlikely to go all the way through. It will have been like that for a long time. Shakes generally appear as wood dries, and are not unusual in the tougher heartwood. In the absence of signs of distress, I reckon 'nothing to see here'.
I trust that your roof hasn't sagged or shown any sign of distress?
Thanks both!
Honestly, we have been quite concerned since the surveyor report came out and just feel anxious. The surveyor did say they weren't really concerned, so i guess that's a good thing.
It looks dramatic, but the shake running through the middle like that isn't going to make much difference to the ability of the purlin to take the load from the rafters and transfer it to the purlin props/gable. No visible deflection in the roof confirms that all is well. I'd be very surprised if one of my purlins hasn't done the same, but I won't be looking for it, or losing sleep over it!