IMO that's a bladder seam imprint or similar artifact from the molding process, not a crack. The creak, if coming from the steering head, is likely due to the bearings vs frame seats. A thin film of thick grease, like moly, on the bearings' entire exterior surface should stop that. Including the races where they touch the fork. After top cap compression and assembly, a brief ride, loosen the stem bolts and re-preload the bearings. Tighten stem. If you're a heavy sweater or wash the bike a lot, this needs to be redone at least twice a year.
Hope this helps,
That helps a lot thank you very much. This "crack" seems to be inside the top tube as well but looks like it has some kind of carbon tape bonded to it. I've just reassembled and greased all bearing surfaces and used some carbon assembly paste on the bar clamp and on the stem where it attaches to the steerer and it didn't creak when I popped round the block so fingers crossed that was the issue but it did concern me when I saw that inside the frame. The only thing that made me think it was ok was the fact the paint on the outside wasn't cracked and i think it would be if this really was a crack