Which compost?

  I've only ever used multi purpose compost(the stuff bought from discount shops) for pots &baskets in the garden,but now i'm growing veg on my own plot i'm a bit lost on the whole compost debate.Peat or peat free, "organic","aerobic"(sp?),& who or what is John Innes.
   Any advice is most welcome.
   

multi-purpose composts, whether peat-free or othewise, do not contain soil in them.  Some have added nutrients e.g. growbags so are best avoided for seedlings, which don't need any nutrients to start with.

JI contains a proportion of soil.  It's not a brand, rather it's a prescribed combination of soil, sand etc.  JI No 1 is for starting plants and seeds off; 3 is for permanent planting in pots of perennials, and No 2 is in between.  It is more expensive and for annual seed-sowing it's a bit of an extravagance.

Re composts, there are a couple of threads on the go in this forum about peat-free / brands / etc.  Have a quick browse and you'll find them.

Peat is a natural resource which is declining in quantity so the aim is not to use it - it's not sustainable.  However the quality / suitability of peat-free for seed sowing in particular is not established, it can be not fine enough.