Hi Nuts,
If the instability/giving way continues I'd definitely go back to the sports physio and get it re-assessed to make sure that there is nothing that was missed the first time or that you haven't done any further damage - to the MCL, ACL, meniscus.
If the chondromalacia is still a problem even with the exercises (assume they include exercises to strengthen the VMO?) and there are no other issues, I'd also consider asking your physio whether some McConnell taping can help you? (It's a specific taping technique to help reduce patellofemoral pain), it helped me heaps.
NNW - chondromalacia patella is a softening of the cartilage that lines the back of the kneecap, often the kneecap doesn't track properly which causes/increases the irritation and pain at the front of the knee/behind the kneecap. Exercises that strengthen Vastus medialis (the quad muscle at the inside front of the knee), help improve the tracking of the kneecap. Taping specifically for this problem can also help, especially until the VMO gets strong enough to do its job properly.
[N.B. I'm not a physio but have a sports science/coaching background, work in high performance sport and have a long history of knee injuries (both knees) - ruptured MCL, chondromalacia, meniscus tears x 2, osteochondral defect on the back of my patella (chunk out of the back of my kneecap caused by a fall), fat pad impingement, ITBFS ... I'm sure there's something I missed!

) . After my MCL tear (many years ago), I wore a brace for that for a while (until it repaired itself), did a stack of knee rehab exercises (including lots of VMO ex's), used taping - different types depending on what the issue was. Walking poles are also one of my best friends when walking, especially for steep descents (more open terrain, obviously difficult to use if it's very scrubby or you're scrambling).
Nuts, I moved back to Perth last December (I'm originally from Perth, I just had a 17 year interlude in Sydney!). However I received an offer to good to refuse when we were in London recently so am heading to the UK to work in January. Will be doing most of my walking there (or Europe) for the next little while!
Cheers,
JB