I may be compeltely wrong here, but surely that not an air/fuel mixture screw given its an EFI engine?
If it is still original cat, as you stated, chances are it needs replacing. Best bet is to buy one and have someone weld up a pipe with the appropriate flanges as buying and old used one...
So had a little run down to C&M: The Bowl on Sunday with my pal Ben in his Polo, plenty of photos below.
Car kept cutting out on me and taking about 10 mins before it would restart. Always happened when coming on boost. Turns out it was my Crank Angle Sensor, another cheapy fix tbh but such a...
Incase it helps, the wire connects to the same circuit as the brake light switch, located in the fusebox in engine bay.
Had to tap into mine as there was a short on the horn side I couldnt find