For many years now it has been a combination of cadets from an extremely good tried and trusted source, together with a roughly equal mix of experienced pilots.
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I am not sure where you get the idea that the vacancies available are "not open to all"? The fleet office is awash with applications, and more arrive with every mail delivery. It is always a case of sending in a current CV, and as so often in life, it is sometimes simply a case of timing.
So I am afraid you are wrong on both counts.
I was talking about what Monarch is taking of pilots this year since you can find the following on the website:
Please note that at the moment there is a recruitment freeze and applications are not being accepted.
It is very hard to see that they are open for applications, but I guess that one should just ignore that and send an application via mail instead.