Hmm perhaps those cheap key fob replacements aren't so worth it after all.
A few weeks back you'll remember that my spare key fob from BMKeys.com failed. The key works fine with the car and doors, just the remote fob died. I bought two spares, one for £30 from BMKeys again, and another from eBay for £6. Both internals appear to be 100% the same, even the antennae routing lines are the same.
Well the eBay one stopped working today, no signs of damage or moisture ingress. The BMKeys one still works, and I'll be using that for a bit now and keeping an eye on it.
Considering a dealer supplied one lasts over 10 years before the sealed battery starts to show signs of getting weak, I think £120 for an OEM key is probably a better investment than having to buy the third party ones multiple times... I will no longer consider an OEM key price a "rip off".