A useful link for all A Level Economics students relating to price leadership http://www.scmp.com/news/world/article/2142410/saudi-russia-alliance-could-spell-doom-opec-cartel-has-ruled-oil-industry
Spec references below for Edexcel, AQA & OCR: 
Edexcel spec reference 3.4.4 d) Overt and tacit collusion; cartels and price leadership 
AQA spec reference The reasons for non-price competition, the operation of cartels, price leadership, price agreements, price wars and barriers to entry.
OCR spec reference P 12 Explain the significance of types of collusion in oligopoly markets