Hi im needing some advice its that time of year again MOT time and guess what its failed

thing is its failed on emissions and mot place said the cat has gone down!

im not convinced considering its been stud all winter and only started once a month for the last 6 months.

i was thinking of just giving it a blast down the local motoway clear it out like.

just had new plugs! leads and coil pack were changed only 2 years ago.

also no warning likes were on the dash.

whats your thoughts?