London Underground must be close to reaching its quota to fill these vacancies. They are trying to strike a balance between number of interviewees already in the process, and the number of fresh interviewees to invite. The last thing they want to do is interview a perfectly good candidate who passes all tests, only to find that there are no more vacancies left to fill. I know it is painful, but there's little point in pressuring LU to interview you. Wait, and let nature run its course. If there are enough fails at AC2, the best AC1 scorers will be invited to AC2. And once this wave is complete and there are still vacancies available, the best SJTs will be invited to AC1 and then AC2.