… Premium without breaking the boiler or any of the plumbing!
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In production the vacuum breaker in an Andreja Premium is secured with thread-lock. To apply sufficient torque to break the seal created by the thread-lock is difficult as the thin-walled copper boiler is not securely fixed to chassis of the machine. It is held in position by the thin copper pipes to and from the pump and the group head. These flex when any substantial torque is applied to the 19mmAF head of the vacuum breaker.
One method, suggested by DaveC, is to place the socket over the 19mm hex head of the vacuum breaker and tap the extension arm with something not too heavy. This acts like a slide hammer or impact wrench to loosen the join. In this instance this did not work and a more "heavy-handed" method was required.
This method was developed by Mole and DaveC (Coffeetime UK forum members).
Brace the machine against something solid to stop it sliding away when torque is applied in the direction of the red arrow.
At the same time apply counter-torque (green arrow) using a large screw driver positioned between the least flimsy looking fixings on the top of the boiler. This process allows sufficient torque to be applied without damaging the vulnerable copper pipes holding the boiler in position.
Once the vacuum breaker was removed it was disassembled and then cleaned in a strong citric acid solution for 30 minutes. This is what a new vacuum breaker valve looks like,

Picture from BellaBarista website.
This is the brass centre pin of the vacuum breaker before it was cleaned in citric acid. The valve is easily disassembled by removing the cir-clip. The splash cup is held in place by the torque of the valve's nut, this was also cleaned to ensure the mating surfaces were well sealed. The vacuum breaker valve was replaced using PTFE tape, rather than thread-lock, to aid removal should further maintenance be necessary in future.
Valve and splash cup cleaned and re-installed.