UK Environment Agency - "Composting and potential health effects from bioaerosols". There are other EA docs relating to Open and closed composting operations both of which reiterate the 250m.
Studies on the problems of industrial composting since 2008 has determined this 250m and other standards depending on the process used e.g. HSE, Sheffield University, German authorties, Parlimentary review, National Aspergillosis Centre, etc. Note: Germany now follows an increased standard of 300-500m. Again, I don't believe the proposed former Norton Aerodrome site is suitable for this activity particularly in light of it being atop one of the highest hills around Sheffield. Hilltop locations have found to increase the spread of biaerosols.
The EA are tightening their permit regulations and increasing licensing costs hence Green Estates desire to close its other sites and create a super site - as indicated at their presentation 19th July.
Many applications around the country have been rejected for several reasons but their locality to residential and workplace property being the most common for public health. To comply with EEC requirements on reducing landfill the government is under pressure to increse recycling, the green waste recycling was seen as the quickest answer but the health risks were not then well known or advertised as is the suspicion. Recently the Mirror newspaper reported on the 17th June 2011 "young woman has made medical history after a double world-first operation to cure her lung condition" following 'multi-resistant Aspergillus'. Alas there has also been a fatality believed to have been caused by the spores from composting. Evidence is growing on the health affects so operation standards are therefore being increased. The legal fight is that the improvements are not being done fast enough in the UK.