When I was this guys age the classic prone firing position using a Lee Enfield No 4 was used.

'LEGS The legs should spread loosely and without tension. The left leg will be straight but the right leg will be bent and drawn up 45 degree in order to ease the pressure on the stomach and stabilize the position. The inside of the right knee will lay upon or against the mat.

FEET The position of the feet is determined by the position of the body. The left foot should turned inward. The right foot should rest with the instep pressing lightly against the floor.'

Firing a BREN LMG was slightly different though and required both legs to be straight with the toes dug into the ground. The BREN had a habit of jumping forward when being fired and the toes dug into ground helped stop that from happening.

The weird thing is though, I reckon I could still disassemble and reassemble a BREN in the pitch dark 25 years later. Maybe not, after thinking about it.