I have just called them and, yes, this is how Thames Water operate. They are sending another man to investigate the leak. Sorry everyone for the false alarm. I did however tell them that this "cold calling" approach is disconcerting.
He has painted a blue arrow pointing towards the house. I have since called Thames Water billing and asked if they thought our water bill was high and she said that we seemed to be using very little water given that there are two of us in the house. Also, I dug the front garden last Saturday and the ground there was bone dry. I am still struggling to believe that this man was genuine.
A man just came to my door saying he was from Thames Water and that he believes we have a leak. I was immediately suspicious and did not let him in. He asked me to turn off the water at the stop cock until the cold water stopped running, which I did and he remained in the street looking into our water meter. He then said that meter was still running so we must have a leak and grinned at me. I did not believe him and said I was going to call Thames Water and check with them. Thames Water knew of no such man so I went to the door to send him away just as he put through a Thames Water card which says " We think you have a leak" with a reference number and information about how the pipe from the road to the house is the customer's responsibility and that failing to respond could result in them using legal powers... It said he was a contractor from PN Daly and Thames Water do use PN Daly. However his van was completely un branded and the reference code he has given me means nothing to Thames Water. Whilst outside my house he received a call from a lady in Crystal Palace Road who was also asking him why he needs access to her kitchen. So - I believe he is a conman. Think twice before believing him.