Thanks for the replies. So I pressed the Mercedes connect button and an advisor explained she would send a technician to my work place. Monday morning, rush hour and within five minutes an RAC technician explained he had been to a couple of other Mercedes C classes with the same problem. The catches holding the bonnet in place on the right and left and if the catch doesn’t err catch then the sensor goes off. A simple five second adjustment of the catch on the bonnet lid with a spanner and all is well. I thought the worse and about the complexity of modern cars with sensors everywhere, courtesy cars and the inconvenience.I would imagine there is a plunger type switch somewhere. Try and find that and push it in and out a few times, then drop the bonnet from about 12-18" up.
I can’t fault Mercedes or the RAC today. Thanks once again for the replies.