Sounds like exactly the missed junction (North West Junction) and the, admittedly rather nice, streamway is "Turkey Streamway". Coming back down an hour or more later the junction is totally obvious but on the way upstream you have to duck under a rock to spot it. The unnecessary extra detour broke one of my companions on a trip where I, like many others before and since, missed the turning
Another cock up we did on a different trip, was doing some kind of loop in one of the boulder chokes on the way out and me and my mate somehow went from the front of the party to the back, and lost the others who still though they were following us. Took us ages to find the route again and find our own way to the entrance.
However the best ever story I've heard from that cave was two student parties from, I believe, either oxford or cambridge, where on the way out the two parties had coalesced into a bigger, and now unmanageable group. One person had done something similar to our boulder loopy thing and ended up left behind. When the rest got to the entrance his original group thought he was with the others, and the "others" of course had no idea he was believed to be with them. They all exited the cave and went back to the caving hut. Unfortunately the gate arrangement at the time was a padlock so not only was the guy left behind, when he eventually found his way to the entrance, he was locked in. So far merely an embarrassing cock-up, however it gets worse, as the rest of the group then assumed he'd get a lift back with the others so all packed up and drove home leaving the poor chap locked in the cave. Apparently he managed to shout to a passing hiker a day later who raised the alarm. He was lucky as he'd have been pretty wet and cold and the entrance passage is draughty so it could have ended badly after 24 hours or more shivering his bollocks off. And on a rainy weekday, there's no guarantee of passing ramblers so he could have been stuck there for days.
I believe you can always open it from the inside nowadays.