Recently I have had loft insulation installed, the "surveyor" maintained
that even though there was no obvious form of ventilation it was not
needed because the roof felt was a breathable membrane, looks like
ordinary roof felt to me.

It looks pretty similar. Are you sure you haven't got breathable membrane?

Second if I need ventilation I intend to put through air bricks at either
end of the gables, does it matter how high they are placed?

There no point in insulating the rafters and then putting loads of
ventilation into the warm loft. The loft itself should have as much
ventilation as a habitable room, i.e. a trickle vent, perhaps.

Christian.