Sow around April-when the seedlings are big enough- thin out to around 5 inches or so apart
You will get a usable crop in about 3 months or so.
Yes-use a good compost
It doesn't really matter-to keep it simple a multi-purpose will do there are usually plenty of offers soon -check out B&Q,Homebase Wickes etc- that is what I do
Try Boltardy. Sow when the soil has warmed up - April, maybe.
Put the seeds in a cup or glass and add some water to cover plus a bit extra because they will absorb the water and swell up. Do this the day before you wish to plant them and have the ground dug, raked and ready. Mark a line with string where you want to sow the seed and draw out a drill with the corner of the rake or a stick, about 1" deep.
Next day drain the water from the seeds. Boil a kettle ful of water! Pour the boiling water all the way along the seed drill. Now, start to drop the seeds on to the bottom of the drill, 1 at a time, about 6" apart. Don't worry, they won't be cooked - the boiling water will have cooled by now, but will have warmed up the soil. Then carefully cover the seeds with soil by gently raking it back into the seed drill and gently use the flat top of the rake to tap down the soil along the drill to firm it.
Wait for signs of germination - the leaves are very small and dark red. Hopefully you won't have to thin them out but can leave them to grow and start pulling every other one at baby beet size. Do not cut the leaves off - they will bleed and do not peel them - they will bleed. Just wash the soil off, put in a big pan of water and boil gently until the peel will pull off easily.
You can start growing a bit earlier by sowing in the same way in 3" pots of compost in the greenhouse - 2 seeds per pot, thin to 1 after germination and then plant where you want them outside when they have grown into larger plants.