In reply to Climbygirly:
In the lakes, the National Trust (as a land owner) specify above 450m is fine, as long as you're also out of sight.
The general rule is camp on flat dry ground near a good water source, which is mainly near standing water, and there are amazing camping spots all over Snowdonia.
There are a few places you should avoid because they are either exposed to the elements or very visible from the road or local tourists, notably around Llanberis and Ogwen, but other than that you will never run out of new spots which are sheltered, deserted and beautiful.
Finally, when you go to the loo, make sure it's at least 50m away from the wayer (running or otherwise) - I teach 'carry out solids' because human waste is very damaging to the delicate soil flora, but others choose to bury. If you do this, make sure it's at least a foot down.
Leave nothing but footsteps, take nothing but memories.