Pretty sure you could get an universal cat for about £90?I decatted mine but it was an old girl on an r plate and I never re moted it!!

I'd think second hand no more than about £50?you could remove the cat from the system and have a look inside.normally,if it's blocked,when you give it a shake you should hear it rattling about a bit,or even see big bits of it moving around in the cat.the car will start to sound like someones put their hand on the tailpipe to check for leaks.