I did a few years ago. Cover off, scrabbling around behind the boiler to find the valve while perched on top of the washing machine !
I remember that - in fact it was probably advice from here :)
The fact I did it, and it has _slowly_ gone back suggests a small leak. For various reasons I have decided this is the time to nail it "properly".
Given that external expansion vessels are hardly hard to come by, it's a source of curiosity as to why boilers have them built in at all.