Thanks for all the input chaps. No luck sourcing a second hand part yet but I've had a couple of offers for the RHS one if I go for the Larinis!
@ Miles, Conaero, Loz, Parisian
Interestingly enough a couple of the breakers I've spoken to today (thanks again Mr P) have said they throw away more than half the cats as they are damaged/breaking down/gone completely. This along with number that the Indy I use has replaced, suggests its pretty common. Since it's so common and the number of damaged engines is relatively low, this would suggest the risk is low. As to if its a risk worth taking, I guess I'll find out how brave I am tomorrow morning! I looked into replacing the primary cat and I asked the garage that did some exhaust work on my Alfa but they couldn't source a cat (size/shape) to replace the primary and didn't fancy the job. But if I can't gather the necessary funds for the replacement Larini's I'll look into asking/begging David at Larini to do the replacement.
@ spydercc2002
I've had Supersprint stuff in the past and been happy (mostly) with it. Unfortunately it looks like doing both sides would be more expensive than the Larini option. I'll see if they have any in stock and if they will do a deal on both sides together.