[grizolsemn]

I cannot remember where I read it, it might be Imaam Malik's Muwwata, or Umar Vadillio's fatwa on paper money, or his book The End of Economics.

Any non-muraabitoon literature?

In the UK we have supermarkets, they have a unwritten agreement between themselves not to have a price war (the cake is big enough for all of them to profit nicely).

If it's really unwritten, and there was no illegal contact between the companies ... I don't understand how the state is supposed to police it, or if they even should police it.

Independents cannot compete with the supermarkets. So we have a oligopoly. Monopolies and oligopolies are not a free market, in fact they emerged in a unfree market through state help

You're saying that an oligopoly cannot exist within a free market? It's a free market, the necessary forces exist to produce an oligopoly. The burden of proof is on you to prove that a free market would never result in an oligopoly.

they also use the national road system with the trucks without paying for the cost of maintenance of the roads

Who says they don't pay for road maintenance? Generally, road infrastructure is supported thru fuel taxation. Everyone pays fuel taxation. And since you are paying taxes of the fuel you use, naturally those who use more fuel (and hence more of the road infrastructure) will pay more in taxes to fund the said infrastructure.

..they get subsidies from the state. So people might think they are benefiting from having these supermarket chains (relatively cheap prices), but the High Street independents have closed and look like ghost towns

Large market chain grocery stores get more in subsidies as compared to independent grocery stores? Proof. Is it not possible that the large chain grocery store acquired a bunch of capital, took a risk, and setup a scaled up business allowing them to consolidate and reduce costs - undercutting the independent grocery store? Perhaps the independent grocery store wasn't as lucky, or not as entrepreneurial, or was happy with their little store and not interested in expanding, or didn't have access to the capital, etc .. (i.e. stuff that happens in a free market).

...so there are socio-economic costs that will impact on the community in years to come. Less cohesion less community brotherhood/sisterhood.

Free markets don't create utopia, but at least it is fair. Utopia is not the goal of Islam either. As far as I understand, Islam doesn't design economic principles to reduce disparity between rich and poor below a certain ratio, nor does it morph the economic environment so that no one company can have more than X% market share ... Islam implements justice, and sort of lets things go as they go ...