The bit of information we are missing which could confirm Tinstaafl's diagnosis is what your mixture was at 9000. ie whether you leaned for max rpm at a full power run up, and whether from that point you were leaning on the way up.
I never go full rich when it is warm out except when starting, and immediately after I "lean for taxi". When leaning during full power run ups, I start to see significant extra rpm at around 3000ft DA. People are taught to go mixture RICH on the descent because it's easier, being too rich is less risky than being too lean, and as most people fly in the UK appear to fly below 2000ft it probably makes some sense.
So at 6500 ft with above standard temps I would expect you to be overrich at mixture full in. Of course this all depends on how your mixture is set by the mechanic. It's surprisingly easy to foul plugs on some engines if you taxi on full rich (of course power is low), and you also burn more fuel when overrich which if you miscalculate could leave you short.
Mixture and it's effect on power/engine damage is not well understood by most, including me I have to say, you really need instrumentation to know what is going on in your cylinders, and gamijectors are superb.
Mixture setting on an uninstrumented engine is an art, but should really be mastered if you fly in the mountains, especially for take off. Having mixture wrong when hot and high is one of the "gotchas" which can cause you distress. You learn something from every flight, and the good thing is you are less likely to do it again.