the anti-vac valve exists to prevent a vacuum forming in the system when the machine is turned off, which is likely to result in 'false pressure' when the machine is turned on next time (i.e. it tricks the pressure switch into switching the immersion element off at an artificially low pressure, and so the system runs cooler than it should)
to perform this task our anti-vac valve is open when it is in the down/resting position, which occurs when the system has less than about 0.3 bar in it. when the boiler has more than about 0.3 bar in it there is sufficient pressure to lift the weight of the centre pin in the anti vac valve upwards, causing the o-ring on the centre pin to seal
if the anti-vac valve is popping into the up (sealed) position, but failing to seal it either has a bit of grit or muck between the valve seat and the o-ring (most likely), or the o-ring has gone hard with heat, or it is nicked/split/cracked/otherwise damaged
if you take a 17mm wrench and a hammer to strike the wrench to shock the anti-vac valve free from the boiler you will be able to remove the valve off the boiler
only once you have removed the anti-vac valve off the boiler should you remove the stainless steel circlip that holds the valve together; the centre pin will then fall apart from the valve body and you will be able to take the o-ring off the centre pin to inspect its integrity
Thanks for explanation, Reiss. It's certainly useful to know a bit more about how things work, particularly if you're someone with zero idea on these things like me.
I've been waiting on getting the correct spanner for the job; however, before going any further, I thought I'd make sure I'd got the source of the leak correct (which, in turn, involved waiting for a cardboard roll to become available). Anyway, I was wrong. The steam is coming from the safety valve.
Now I've looked at some of the older posts about resolving this, which suggest I'm best having a go at trying to get the valve to close before I think about removing it. Bearing in mind that, like I say, I've no experience doing this kind of thing, how do you recommend I go about this please?