Took off the mushroom to lube the flow control valve and noticed the big o-ring at the bottom of the mushroom (made of EDPM, I think) has split, like a thin ring was cut off it all around:

Common sense (+ this video + this diagram from an old page by Dave) is that the o-ring creates a permanent seal between the mushroom and the thermosiphon chamber around it, that it is stationary and that no parts move to push against it during operation (correct me if I’m wrong please). So is it just the water pressure, in combination with the heat and rubber hardening over time, that split and cut the thin ring off the O-ring like this @DavecUK ? Also, what would be the effect of this o-ring failing to create a seal? Would it be loss of pressure during the shot? Or something else? (in other words, how would I know it failed without taking the mushroom out)?

While I was at it, I noticed that the Viton o-rings that Dave recommended for the flow control valve are indeed holding up much better than the original green EDPM ones I had before (the old green ones split in a similar way to the big o-ring pictured above, while the new black Viton ones are holding together):

I therefore decided to get Viton O-rings for the bottom of the mushroom as well, for next time it will need to be replaced. If anyone is interested, the size is BS210 (Inside Diameter 18.64mm, cross cut 3.53mm). I got a pack of 10 for a very reasonable price here.