As others have said, this hangs on whether your site is deemed a 'congested area'. If your site is defined as such then you need permission, if not then you don't.
Someone gave me a definition which I thought would be pretty arguable in court, namely that a congested area is anything in yellow on the 1/2 mill map. But recently a FIE(H) told me that the CAA argues that a golf course is a congested area ! Figure that if you can.
Best to get a ruling to be on the safe side - Call Bob Jones (AOC and PAOC operators) or Keith Thomas (Gen Aviation) on 01293 573528.