Thanks for the replies, I'm beginning to think perhaps we did'nt cause the marks in the first place and that he noticed them after our cleaner had gone in but had been caused by something like dragging a pushchair on the carpet with the wheels locked!!! The marks are at the top of the carpet fibre and not all the way through the fibre, so no cleaning solution will dissolve the marks as it looks as if the black bits have stuck into the top of fibre.
Any suggestions on handling the client on this one? Could it be our fault, although speaking to the cleaner she is not aware of leaving any marks, as she told me that if it came off the nozzle the line marks would have been approx 2 inches apart and not the 9 inches as is on the carpet or could the client be trying to pull a fast one and how do I tell him this