Hi Darran, just to share most people think that bleach is the best way to tackle mold growth lichen etc, bleach will not kill mold , it merely bleaches it, that's right, not only will mold survive a good bleaching , it'll look as though it's gone, lulling you into a false sense of security, bleach is a disinfectant and not a cleaner and doesn't really clean dirt and stains from surfaces, its best to just pressure wash the ingrained soils from the patio slabs, to remove the black lichen spots properly you'd need to scrub them, the most effective tool i have come across for these little buggers are stainless steel scouring pads not the ones you'll find in supermarkets as they are only 15 gram, go to a janitorial supplier and get 40 or 60 gram ones and they will eat through the toughest of lichen doesn't matter how old they are, i know it sounds crazy because you will have to get on your hands and knees to do this with the s/s pads, this leaves you with two choices as all you need to use is water with the stainless steel scouring pads to remove these black lichen spots, you have to educate the customer about the problem, then you can justify the extra cost if you end up scrubbing them, or you ask the customer if they want to carryout the scrubbing themselves if you leave them with some s/s scouring pads, one 40 gram stainless steel scouring pad will cost £1 and will last long enough to remove 100s of these black lichen spots, i know many guys will laugh at this solution but it does work and to spend £1 won't hurt to find out.