My 52 plate GTV TS (109k miles) is also borderline/fail on emissions at the moment. HC and CO are high on the fast idle test (higher revs = higher values).
I've found a series of videos here Kanal von TennecoInc - YouTube
which go through various diagnostic tests, some of which you may be able to do, some are more involved. Could be useful, especially the simple temperature test to check if the cat is actually doing anything at all. Also, in looking for cats, this is the cheapest I've found Cats Direct | BM91292H
(from the picture it looks like it should fit - no idea if it's much good, but hopefully will at least get me through MOT next month).
I have read (whether it's true or not) that the two pre-cats aren't really necessary overall for the MOT. Apparently they heat up quicker (being small and right by the engine) so get to work on the emissions before the main underfloor cat is up to temperature. Since the MOT emissions check should be done with it all warmed up, the underfloor cat should be enough (assuming everything's running well, I suppose). Certainly Alfaholics sell a full stainless system without pre-cats which they say is MOT compliant...
Mine was in today for service and the chaps reckon the little silencer behind the cat may be partially blocked (highly technical "see what sound it makes when you hit it" test), which could affect things.
The two lambdas on the front pipe are responsible for letting the ECU know what's going on, so they're the important ones if you've got running issues (aside from other potential causes anyway). The single rear one after the cat is only as a check on the cat function/emissions itself, so won't affect running, but will turn on the warning light. I called Alfashop last week and they're doing the proper Bosch ones for £60-66+VAT.
I might do some further tests on mine over the coming week or so; I'll update if anything shows up that might be of some use...