There's a comment about this in the latest GASIL (it's available in the Internet from the CAA site
http://www.caa.co.uk/publications/pu...on=sercat&id=7 ). It says that A/G frequencies are only designed for use up to 3000ft AAL. If you use them above this then there could be confusion with other aerodromes. Personally, I'd give them a call if I was slightly above this, but it might be best to include the aerodrome name in each call to avoid confusion with nearby aerodromes on the same frequency. I've certainly been confused by an aeroplane at Sywell when I've been approaching Compton Abbas.