Farmer's advice: The most rotted manure is the best, and in this case can be most easily found under where the is grass on top. Of course for those who like a challenge there is on doubt a motherlode deep under the biggest steamy mass... It really is a case of all that glisters is not gold. Even though as an enthusiastic debuttant the temptation may be to go for the steamy fresh stuff and be proper authentic, the ready to go will have composted to be more the colour of earth.
It's horses for courses time: Having to pay for compost every time would become uneconomical so hey, try to find a friendly farmer. There are plenty about.
Its quite a job but its getting done. Want to get the digging finished this week then can kay it out and let it settle before laying seed down for grass.