What compost mix to fill a diy raised bed on table with?

[The Griff]

I have a steel box section 8' x 4' table that I am planning to turn into a raised bed 8 foot by 5 foot and about 15 inches deep. I am building it on the steel frame so the complete thing will be sat about 3 foot high, to keep the dogs off it and to get a bit more sun as it would be in full shade if it's any lower.
It will be a bit like one of those veg trugs. I plan to grow carrots, lettuce, spring onion and maybe a few herbs in it with some strawberries maybe overhanging down one side.
I am going to line it with a sheet of plastic after making a wooden outer frame out of planks so the water can only drain from the bottom.

My question is: What would be the best thing to fill it with, would the big 120 litre bags of b and q multipurpose compost mixed with a bit of sharpsand be ok with some water retaining crystals or should I look at adding some topsoil or peat as well. I can get compost allot cheaper than topsoil.

[arpoet]

The problem with MPC is that it will soon lose all its nutrients. Maybe ok for the first year, but will need some manure mixing in.
Myself, i would start with cut up trigs in the bottom then rotted manure mixed with leaf-mould and shredded paper then topped off with MPC/sand mix. But the mixture then would be too rich for the carrots.

[maverick451]

I really like the B&Q compost, its good stuff.

Ive used a mixture of 3/4 B&Q compost and 1/4 of well rotted manure (again bought from B&Q)

[The Griff]

I have set the boards up, they are 6 inch wide so am going to go 12 inch in depth.
I have just noticed aldi have 40 ltr bags of soil conditioner for £2 so maybe plenty of rotted manure mixed with a mixture of compost and whatever other soil I can get mixed with half a dozen bags of soil conditioner, what about some peat as I can get some of that pretty cheap just to add to the mix.
I can always leave on strip Down one side free from manure for my carrots.

What are the advantages of peat and why are some composts peat free, why would you not want peat in your compost?

[Penellype]

From a planting point of view peat is great, but from an environmental point of view its extraction is destructive and it is a limited resource.