Emmissions Warning Light on with fault code 16804

Hi guys, I've been on here a while although not said a whole lot as I don't really know anything , but I was hoping someone could help me.

I have been having problems with my emissions warning light coming on my 2003 Octavia 2.0 ambiente.

It first came on back in January, in the snow 10 miles into a 220 mile journey. I completed my trip and took it into a skoda dealer later that week. £35 and 30 mins later a chap came back and said it was a fault code relating to a problem with the cat, which he had tested and was working fine. He then said that meant it was probably a lambda probe spiking but it would be expensive to replace without knowing which so I should return if it comes on again but ultimately I would be best just living with it.

Back at the end of June, on one of the hottest days of the year, it came on again about 10-15 mins into a journey. I left it for a while this time before getting a mate with a cheap ebay cable and some free software to check and clear the code. It came out as a 16804.

A couple of weeks later, it again came on about 10-15 mins into a journey. This time I bought myself a cable and checked it with VAG-COM today. I got the same code and it said:

Catalyst System: Bank 1: Efficiency Below Threshold

Now, if I log the outputs of the lambda sensors, would I be able to tell which is faulty? I also read it could be an o2 sensor instead, is that plausable and if I logged these outputs could I tell what's broke?

The car's done nearly 108k miles and is still on the origional exhaust so its not too surprising something's gone on it, especially as this is the first problem I've encountered. I'd really rather fix it than leaving it as I want to get the car into a possition where I'm happy to sell it incase a decent VRS appears for sale in the Peterborough area....

Thanks,

Rob