The legal situation is different in Scotland than it is in England and Wales.
In England and Wales, there is no legal right to camp 'wild' and, strictly speaking, permission should be sought. But this can be impractical.
However, in high mountain areas, and in most other hill land, wild camping is generally accepted above the intake walls, providing it is for a limited time and its fairly discreet - out of sight of houses/farms.
There's always a slim chance that you'd be asked to move on.
In Scotland, the current access legislation - (early 2005) - is explicit about people's right to camp wild on hill land - usually more than 100 metres away from a public road - although you'd probably want to be much further away than that!