So I went for a cheap MOT test - £30. Figured it'd be alright as the car has only ever had 1 advisory of note (battery clamp wasn't in place) so you can imagine my surprise when they told me it had failed 
I was even more surprised when they told me it failed due to the cat! They said they don't think it's a cat as it doesn't have a stamp or marking on it and were surprised when I told them that it's a sports cat.
Emissions are spot on.
So they said either I can fit the OEM cat or they can take off one side of the sports cat (at cost) and they can have a look. I figuered I'd fit the old cat myself and take it back but for some reason I can't find bolts that fit (M10 is slightly too big, M8 is a way too small). But now I'm thinking that if the car has passed emissions and the rest of the car is fine, the exhaust isn't crazy loud as it would be if there was a straight pipe in there, then surely the onus is on the garage to prove that there isn't a cat in place? Why should I have to pay more because they can't work it out?
Just wondering how best to proceed with this? I'm hunting for some bolts but wondering if it's worth have another conversation with them - guessing they won't budge though.

I was even more surprised when they told me it failed due to the cat! They said they don't think it's a cat as it doesn't have a stamp or marking on it and were surprised when I told them that it's a sports cat.
Emissions are spot on.
So they said either I can fit the OEM cat or they can take off one side of the sports cat (at cost) and they can have a look. I figuered I'd fit the old cat myself and take it back but for some reason I can't find bolts that fit (M10 is slightly too big, M8 is a way too small). But now I'm thinking that if the car has passed emissions and the rest of the car is fine, the exhaust isn't crazy loud as it would be if there was a straight pipe in there, then surely the onus is on the garage to prove that there isn't a cat in place? Why should I have to pay more because they can't work it out?
Just wondering how best to proceed with this? I'm hunting for some bolts but wondering if it's worth have another conversation with them - guessing they won't budge though.