there will not be transmitters at the tunnel mouths - the tree thinning is routine work.
A trial mobile phone service will take place at one London underground station during 2006. If the results of this are satisfactory the service will be extended to the whole of the underground system by 2008. At first, mobile phones will only work inside the stations themselves - in the walkways tunnels and on the platforms. Later, if public reaction is favourable, mobile phones will work on the trains as well.
The way it will be done is by using picocells - small, low-wattage, short range antennae about the size of a smoke alarm. These will be dotted around the stations, and will be connected via cable to special masts above ground - probably on the station roof. The picocells are very low-powered, only about 1 watt each, which is perfectly OK for the purpose.
Later on, if things work out, you'll also be able to access broadband internet connections on the trains - but that's not going to happen until after the phone system is up and running.
One big concern, and it's been voiced by several experts, is that a mobile phone service would potentially enable terrorists to use phones as a means of remotely detonating explosive devices on the trains.