According to the Telegraph the performance of peat free composts are, to say the least, variable. A recent article reports on testing carried out by gardening Which. The conclusions are that some peat free composts are not worth buying.
The best for seeds were: “… B&Q Multipurpose, B&Q Sowing and Cutting composts and New Horizon Organic and Peat Free Growbag.” And for small plants “… B&Q Multipurpose, B&Q John Innes No.2 and Westland West + Multi Purpose Compost.”
I have used bags of “New Horizon” for seed sowing for many years with great success. Last year I was alarmed to find what looked like the remains of ground up kitchen units in most bags. There have been other similar reports about not so natural foreign objects in the compost. I have not found anything from the makers (William Sinclair) to explain what happened.
When growing from seed it is really essential to produce strong, vigorous plants as they are more likely to thrive and resist pests and diseases. For that you need good compost either commercial or DIY.
Some people make their own seed compost mix but to be honest the one time I tried I got severe damping off – a fungal disease that rots the stem where it enters the compost and can fell a whole tray of seedlings over night.
If you are interested then try a mix and see how it performs. Some suggestions: mature leafmould (~ 2 years old) is said to work on its own but can be mixed with sterilised (to avoid damping off) loam at a ratio of 1:1. Or garden compost, leafmould and sterilised loam at 1:1:1. [From “Encyclopaedia of Organic Gardening”, p.116, HDRA/Dorling Kindersley, 2001, ISBN 07513 33816 ]