Price of £1.99 is for one mica sheet 

more than one quantity pick includes discount

A microwave mica sheet (or waveguide cover) is 

a crucial, heat-resistant, and insulating component that protects the Magnetron from food splatters and moisture. 

If it turns black, burns, or sparks, it must be replaced immediately to avoid damaging the microwave. These sheets are inexpensive and are easily cut to size

Why let your Microwave fail when you could save it for as little as £1.99