Having worked perfectly my Sat Nav on Octavia 111 has developed a fault. I switch on and the map comes up. I key in destination and select route preference but the Sat Nav then goes off and displays "please insert a data medium with valid navigation data" Ejecting and reinserting the sd card makes no difference. 

Is there a fix or has the sd card perhaps been corrupted in some way and will need replacing?

Media can become broken.  You could download latest maps and try, or ideally run a bad sector scan on the map card.