I have a property built in1892, the brick walls are an English garden bond, on removing blown plaster I have found the stretcher brick layers to be OK but the header bricks lack mortar so much so that a tape can be inserted a good eight inches, I think the tape only stops as the external brickwork is rendered and painted. I would like any advise on the best way to insert fresh mortar, would a 3:1 mix be suitable and should I insert a silicone layer first to aid damp proofing?