V reg Ranault Clio Speedo Fault

Hi, I have a V reg Renault Clio and the speedo works when you start driving but then later on into the journey it just stops working.

I had a mechanic come round and he looked at it, put his hands somewhere behind the engine and said it was loose and that it was all fixed now, went out for a drive and same thing happened, but he did say if it happens again then i need to buy a new sensor.

Are new sensors easy to come by and cheap or expensive and are they easy to install, anyone got any tips that are going to help me?

many thanks