What is the ruling on taking money offered by the landlord for the electricity bill, which he refused to accept payment for, given that the rent is shared between me and my flatmates?

It is obligatory for you to return the money to its owners unless they permit you to possess it. You are merely an agent for them in delivering it and paying the bill. The man did not explicitly state that the amount was a special gift for you; rather, he refused to accept the bill amount and instructed you not to pay it. Thus, the right of your colleagues has been forfeited, and it is incumbent upon you to return their money to them. An agent is trustworthy concerning what he receives and disburses.