I may be being thick, but I don't see why the Lambda fault codes are related to the possible gasket failure/leak, unless you have such awful compression problems that some cylinders are not firing, in which case you'd certainly know about it from the driver's seat.
As others have said, I'd start with a compression test. The rear bank is a pain, because you have to remove the plenum, but it is not hard. After this, you will know what you are dealing with: front bank, rear bank or both (or none).
If only the front bank, I'd just get that done. If it involves the rear bank, it gets more involved.
If you are going to keep the car, Pud's suggestion of whipping the engine out and doing a proper job on the cylinders, belts, clutch is probably the right answer. £2K or so, and you're done for several years.
If you're not planning on keeping the car, then I'd sell it with the fault. You won't get your money back on the work done if you fix it.