I'm not sure about the UK, but here in the States, practically all of the nurseries, garden centers, and larger hardware stores sell "Composted Steer Manure", "Planting Mix", and/or "Compost"--not to be confused with "Potting Compost", which can be used, but is expensive. All are suitable for bumping up the organic content of the soil, and the manure and compost will also add living soil organisms. To add essential nutrients to the soil, I would add small amounts of blood meal for nitrogen, bone meal for calcium and phosphate, and larger amounts of seaweed for potash. Note that a lack of organic matter or nutrients rarely prevents seed sprouting, but can stunt or gradually starve the seedlings. What were you trying to grow, and what was the weather like when you sowed it?

Never heard of "Composted Steer Manure" but all the GCs sell a variety of soil improvers and composts - as Tugb. states they are not cheap though! Is there no livery stable near where you live that you can buy manure from? If the crops you sowed didn't grow did any weeds or was the soil completely barren? If it has sat unused for years it can take a couple of years and a lot of humus rich material before the soil is good and fertile again. Also did you completely remove all the scrub and bushes?

Rather than spreading a limited amount of compost over the whole plot I mark out where the rows are going to be, dig a shallow trench and put some compost in the bottom of this. Seeds are then sown above this. I should be looking at doing this from now but the soil is too wet.

Seeds ought to germinate, even if the soil is poor. And the best way to regenerate a soil is to leave it to the weeds for a few years, after all that is what 'lying fallow' means.
Agree with Bulbaholic on this, put what compost you have under where the plants are going to grow.

Many thanks for all the answers.
I sowed a wide range of seeds: carrots, cabbage, beans, salad crops, potatoes.
Yes, weeds grew in profusion!
Yes, we removed all the scrub and bushes, firstly with a mini digger, then by digging.
I think I'll take on board Bulbaholic's suggestion and put compost in trenches where I intend to plant. That will cut down considerably the amount of compost I will need.

One of those cases where I would love to look at the soil close too, ie be in your garden with you.
All you can do is try again. Best of luck.

Even your potatoes did not grow at all? Now that is odd. As Owdb. says all you can do is try again, wait until the soil is warm to sow, if you are sowing direct, as seed sitting in cold damp soil can rot way. With the cabbage and beans you could start them growing in the greenhouse if you have one, in trays for the cabbage and small pots for the beans.

Seeds are " funniosities" with their own set of rules, Canalhopper. One year I sowed parsnip seeds and I reckon every single one germinated and grew well. The following year I sowed two packetsful of the same variety and we got one single, sad seedling. The same sort of thing happened with beans and spinach. It's very unfortunate that absolutely nothing came up for you, but I would guess that will have been weather-related rather than anything to do with your soil. Last year I sowed and grew a magnificent crop of Brussel sprouts - this year, the same number of plants, the same treatment, lots of beautiful big plants but ALL the sprouts are tiny, misshapen and leafy, rather than hard and firm. Very disappointing. MG is right about starting things off in a greenhouse where possible. Have courage and try again. You will be pleasantly surprised, I'm sure.

I am surprised about the potatoes but could the problem with the seeds be due to the original quality of them? Were they old seeds that you have been saving or 'cheap stuff' that may be old or have been stored in a hot or damp environment?

Thanks again for all the help--you really are a great set of friends!
I sowed first earlies and main crop potatoes. First earlies did well, though were not abundant. The main crop produced nothing.
All the seeds were good quality, some were from the previous year, but were still within the use by date. I stored them in their packets, in a shoe box in the shed.

Since the weeds grew, I would have expected at least the salad crops to have sprouted, since many of them are descendants of weeds, anyway. Maybe that is the problem--you (and I!) might have pulled them by mistake! Note that the shed is one of the worst places to keep seeds, since there is usually too much humidity and temperature variation to allow the seeds to sleep in peace. A cool, dry room in the house is better, and the refrigerator, in a sealed bag, with an envelope of silica gel or powdered milk to keep them dry, is about the best that a home gardener can do.

I think it was the slugs that ate your seedlings. It happened to me. I didnt go down the field because of the rain. Went down after 5 weeks after it stopped. Result nothing growing under the cloches I had so carefully placed. Best to grow all your Brassicas, Parsnips, and Beetroots in the greenhouse where you can keep an eye on them.
Good luck in 2013.