Martin
No the headset would leave the airplane with me and be stored in a cloth bag in my apartment (in a ventilated cupboard); no not exposed to UV.
It is typically between 25 and 32C here in Barbados - seems that the heat got to it - it looks pristine apart from the plastic around the jack plugs and the "Y" junction. All of the plastic around the plugs has crumbled away and the "Y" is going the same way.
The plastic gets a little soft before it fails - it shouldn't have done that in seven years.
When I tried to get it repaired when up in Florida not where could do it, but they all seemed to know about the problem - apparently the a DC characteristic, the other being losing a gel seal (my other newer set has that problem) "most people just buy a new headset" is what I was told.
You are also in Dublin - so thinking 30C average temperature not an issue
Mine looked brand new up to sometime in year five - other than these three plastic bits the headset does look perfect.