Actually, if I want to be real, there are many unaccountable fueling stations. But what I observed is that lot of people are going into the business and its not stopping. Even in the state I reside, we have may of them. To slim the numbers down, in my environment atleast we have nothing less than 25 fueling stations. Hope I am able to answer your question to a extent?
@livinguktaiwan

Then secondly,

Also, would be interested to find out whether bribery may impact it's effectiveness as I heard the situation can be quite bad in Nigeria. Would be good to hear your take on this. Thanks

On this part, I will like you to make clear what you really wanted to have idea about. "Bribery may impact its effectiveness" in what aspect if I may ask? But I will give you the little I can. Nigeria practises mixed economy, this means there are private owned business and public owned business. Nigeria as a country produces petroleum that is oversee by the Nigeria National petroleum corporation charged with the responsibility of producing and distributing to privately owned fueling station making sure that there is uniform market price in the whole of Nigeria. Meanwhile, that can only be done via proper supervision but you know bribery and corruption can take its place if most of these private fueling station owners bribed the government officials that come for supervision to check the amount the fuel is being sold so as to escape prosecution.

Majorly, the problem we are really having in Nigeria is assigning competent personnels that will supervise project without being bias.