This thread was started back in August.
It seems that Which? have been researching the issue, systematically. Their results are published in the January/February issue of Gardening Which. I haven't seen the full report, only a summary of their results...
27 brands were tested, for seed-sowing and raising young plants.
Three brands have been selected as 'best buy'. They are all peat-based...
- Sinclair’s J Arthur Bowers’ Seed and Cutting Compost
- B&Q’s Verve Grow Your Own Growing Bag
- Verve Seed and Cutting Compost and Verve Multipurpose Compost.
Next, also recommended, though not 'best buys'...
- Westland Jack’s Magic all Purpose
- Westland Gro-Sure Seed & Cutting
- Levington John Innes Seed compost.
Best peat-frees, but not as good as the peat ones...
- Miracle-Gro Expand n’ Gro
- Vital Earth Multipurpose compost.
If anyone subscribes to the magazine they might have some more details, about how good, or bad, all the others were. I don't know which brands came bottom.