I have had anew gas and electric cooker fitted I was told that the back of the cooker should be tiled and made safe is this true 

Do you mean the wall behind the cooker as oppose to the back of the cooker itself?

What is the wall constructed of and lined with currently?

Tiling or other heat insulation behind cookers is common as the heat from the appliance will, over time, damage and degrade a standard plasterboard and stud internal wall. 

Also certain linings on walls could be a fire risk if subjected to prolonged heat.

You don't have to use tiles, however whatever is there should be non combustible.