I'm sure it's in the building regs somewhere but I couldn't find it when I looked just now. This site suggests...
http://www.callde.co.uk/safety_regulations.htmQuote: "A Minimum of 750mm Clearance is Required Above a Gas Hob and 650mm Above An Electric Hob To a Cooker Hood. The Gap May Be Reduced To 727mm Above a Gas Hob But The Hood Must Be Cleaned Regularly To Prevent a Fire Hazard. Wall Units Either Side of A Hood Must Be At Least 366mm Above. A Minimum of 300mm Clearance is Required Either Side of The Hob If Installing Next To a Larder Unit or Flammable Material"
I believe the usual problem is the use of 60cm wide hoods over 70cm wide 5 ring hobs - which can mean the wall units either side encroach.