Don't forget that, despite the government spin by the Daily Telegraph, the market study is a study into the operation of the market for station retail outlets, of which the prices they charge is a result, not the thing specifically being investigated. As per the consultation on the market study, one of their main issues is the Landlord and Tenant Act 1954 which grants businesses security of tenure. This means that once a business has a lease it can't be evicted except under certain strict requirements, even at the end of the lease, and so those with an existing lease stay put, and new businesses can not enter the market. One of the few ways that can change is if the station is redeveloped (i.e. the property subject to the original lease no longer exists).
They are looking as much on the operation of the market for new operators, and its effect in reducing Network Rail's (and other station owners') income, as the effect on consumer prices. The latter is very much a knock on effect of the market for leases.
It will take them up to a year to reach their conclusions, and if it is recommended that the Landlord and Tenant Act 1954 should be abolished / replaced that will affect much more than the railways and will, if ever done, take years.