This is an interesting topic. I know of one sheduled ancient monument which doesn't seem to have featured on TMA and which for all intends and purposes just looks like a 'green' on a housing development next to the Rushy Platt Nature Reserve in Swindon, Wiltshire. According to the EH plaque it is a round barrow and the fact that it is sheduled has prevented the land being built on. This doesn't ever seem to apply to the many Cotswold long barrows that, although marked on OS maps as ancient monuments, have often been ploughed out.

I did notice recently that most English Heritage information boards at ancient or historic sites now have a sentence prohibiting unauthorised metal detecting.