I figured out the Blenheim. First, if the following method doesn't work press control+O to set the turret from the low drag cruise position to the firing position (i don't remember if it needs this step, but if the turret is unmovable that's your best bet).
Then, there are two controls to use. Disable the mouse cursor (default key is F10) to move the gun around and go to shift+F1 view to look down the sights. The gun moves up/down but it has a very limited amount of left/right travel in relation to the turret, so you have to move the entire turret left/right. This is done by mapping keys to move the gun mount, similar to the Stuka and the top turret in the He111.
I use the arrow keys for that, so pressing the left/right arrows rotates the turret and using the mouse moves the gun inside the (severely confined) range allowed by the turret's gun mount.
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