Buy SODA CRYSTALS from sypermarket, mix with water as instructed to make a strong wash, wear goggles and gloves.
Spray on with a garden sprayer and/or brush bad parts
Leave for a few hours or next day, wash with hose or power washer.
Should see you right for a year or so.
Power...
Decopierre
Stone-Coat
Stonefxwalls
Imipierre
serach these for examples
try you toob for lots of samples.
Saw a shed done as random stone there somewhere and a polystyrene obylisk (is that how you spell that????)
Normally you could sand blast, but I would look at SODA blasting, same effect with less damage to the bricks. Less mess to clean up too. End result could still be a bit messy and re-pointing needed.
Other option is to use a brick effect render finish as just posted on here. StoneFXwalls can...