Recent article on the BBC website
To summarise if you are supervising a learner:
Falling asleep is not allowed – The law states that somebody supervising a learner driver is effectively in control of the car.
The AA/Populus survey suggested drivers were also breaking the law by drinking, sending text messages, or failing to wear their glasses while on practice runs with learners.
Some 23% of respondents did not know they would be breaking the law by using a mobile phone while supervising a learner.
And 13% were unaware of the need to wear glasses if they used them when driving themselves, while 9% did not realise that falling asleep in the passenger seat was illegal.
Also the law changed in 2000 and you are not allowed to supervise a learner in any public place (i.e. local supermarket car park) unless there is insurance in place on the vehicle. If spotted by the police both the learner and the supervisor could receive 3 points on their license.