Airfare searches are computationally demanding, particularly when we start adding stopovers. Matrix (and other air fare searches) sometimes need a bit of help to find the best prices. Essentially, you need to narrow down the problem.
In this case you know that you want to fly on a CZ fare. So tell Matrix that you want to fly on CZ flights. Also, KLM will be operating your feeder flight to Amsterdam, and Air France and KLM are the same company, so you should probably widen that to include any of CZ, AF and KL.
Use the Show Advanced Routing Codes link on the search page to show the extra boxes to type in the route codes. The fare search I entered looked like this.
CZ,KL,AF means exactly one flight, with a CZ, KL or AF flight number.
CZ,KL,AF+ means at least one flight, each of which must have either a CZ, KL or AF flight number. Note the plus sign on the slices between AMS and MEL, since of course you will be stopping at a CZ hub en route.
This did give a result including your desired fare, but only in one direction. In the other you are on a slightly more expensive U- fare. Your desired fare L2KRCGB is there, although it now has the 75 EUR surcharge for the stopover added to it.
I notice it is possible to knock another £250 off if you are willing to travel via CTU and CAN on the Tuesday on an E- fare. However that may introduce visa complexities.
It is actually possibly to tell Matrix that you will only accept a specific fare, in this case L2KRCGB. To do this, add the expression
f CZ.CWL+MEL.L2KRCGB into the Extension codes box for every flight.
However, if the price is not available, you will simply be told "No flights found", which does not reveal whether the fare was invalid for the proposed itinerary, or whether the fare would be valid except only that there were insufficient seats in the appropriate buckets.
But if we search once with the Only show flights and prices with available seats option turned off and once again with it turned on, we can infer whether it is an availability problem.
In this case, it is indeed possible to price the whole itinerary using the L2KRCGB fare, but only with the Only show flights and prices with available seats option turned off. Although that information may appear useless, it does in fact validate your earlier understanding of the fare rules, which may be helpful on your next trip.