Sorry to burst your bubble, but that is nowhere in the rules of the road. It's merely seen as good manners by us Lycra clad accountant fatties to go into single file when it is safe for the car behind to pass. Not on the apex of a blind corner, like.Im not an expert but doesnt it say that that cyclists are allowed to cycle two abreast unless it is not safe to do so or if they are restricting traffic. Even if a single car comes up behind them then they are obliged (by law) to reorganise themselves into single file thus removing the restriction that they have created and allowing the faster traffic to proceed unhindered.
they are then free to resume the two abrest formation and continue their chat if they wish.