Just to complicate things, I have looked on the Internet and discovered that mobile phones use a system known as TDMA (Time division multiple Access) which splits a 30khz channel into up to 8 time slices with each user working the channel being allocated slots as required. Now at this point I think my scenario about the Hatfield Tunnel and the ability to use a mobile right through is even more complex and at this point I am going to retire to somewhere that sells headache pills !!