A little more detail to answer the questions above - The survey appears in the current Jan/Feb 2013 Which? garden magazine. The survey was conducted from spring 2012 to find the best buy composts for growing seeds and young plants.
They tested 27 widely available composts - 18 peat based, 9 peat free and asked mystery shoppers to buy them from 4 different regions in the country.
First they tested the handling and texture of the compost for its suitability for small seeds.
They then tried to raise 12 pots of marigolds and basil seeds from each compost ( 3 from each bag bought from the 4 different areas in the country) Apparently basil does not germinate well if there is a high level of dissolved nutrients in the compost and marigolds will not thrive if the nutrient levels are too low! They counted germinated seeds and the size and quality of the seedlings.
Then they tested for growing on young plants and seedlings - 20 snapdragon and 20 tomato seedlings were potted up in each compost. Again snapdragons struggle if nutrient levels are too high and tomato don't thrive if they are too low. They were grown on without feeding until the largest were ready to be planted out. All plants were rated on size, quality and leaf colour.
They state that although grow bags are not intended for using to raise seeds they tested them as some people do use them for this purpose as they are so cheap.
Brands tested included Westland, Bulrush, Verve(B&Q) New Horizon, Levington, Miracle gro, Homebase, Vital Earth, Arthur Bower's
The best peat free compost was Miracle - Gro concentrated enriched compost with a test score of 73% with Vital Earth multipurpose compost 2nd with a test score of 65%
Hope this helps