Yep Brian, GAP's were the same for the Vickers, Mortars and Bren. Rare as rocking horse manure if you've got any
You're right about the fixed line sight BAR. The leveling bubbles were to set the gun to horizontal (Calibrated using the sight clinometer first and.... and...... jeeeees!), set the elevation on the sight for the range from the trajectory scales booklet and elevate the gun to suit. No wonder the Vickers and Mortar crews did a 6 week course!
I learned JUST enough about indirect fire to be able to waffle a bit, not look tooooo stupid, nod and agree at all the right times and most importantly, look interested.