Thanks, good to learn, rivet on steroids was not that far out. - AndyAs Pinky has suggested, dremel a slot or if accessible then you may be able to use pliers or mole-grips to remove.
Andrew - It's a shear bolt, something similar to this: https://www.skoda-dily.cz/nahradni-dil/n90584501-sroub-s-trhaci-hlavou-m8x20-skoda-22166.html
if you want to replace afterwards then just use a normal M6 fastener to secure the ECU.
It's not an A2 though....no need to remove the cage, there are two handle type clips on each side, just pull those to the left and right and it will be released from the cage, undo the plugs and hey presto, you will have the ECU in your hands. that's what I did on my TDI 75 2004 PLATE,