I am not familiar with Heather brick, but muriatic acid typically will not damage brick.
First chip off any bigger chunks of mortar (as much as you can). Spray down the wall with a water hose. Scrub with acid a stiff bristle brush. Nail a brush to a stick to get to the higher portions of the wall. Wear goggles and rubber gloves. If you have any recent cuts on your hands, you may wish to wait a week before cleaning the bricks. Rinse the wall thoroughly with water.
I don't think there are many of our forum members in the UK. But around here, I would go to someplace like a Home Depot for muriatic acid. A construction supply store might have a better price if you need a lot