That's rotten luck. Bet it was some pikey, you see them driving round in old Transits with bikes etc in the back that they've blatently stolen. Cat's are worth a fortune at the moment.
I've just broken my GTV for parts, and only sold the cat last week. I got £50 but the bloke weighs them in so it was probably worth at least double that!
There's two lambda's, I think one is in the manifold and one in a front pipe. I know I have at least one of them, I'll check for you. I do have one of the front pipes as well. They were both new only a few months before I broke the car. I had to chuck one of them though as it caught a big speed bump and wrecked the flexi! So I can only help with one pipe.
PM me if you are interested. If you give me your email I can email a photo of the front pipe / lambda.