Spray with 3/4% hypo, leave for 20 min and light pressure clean. You'll obviously have some small loose bits flying off but they would come away even if you used a stiff brush. Then I'd spray it with anti fungal wash.
Walk away from it ...it's knackered.
I have been pressure washing for a year but have never come up against tarmac...could someone suggest how to go about it please?
Doesn't look to bad and the customer will be happy when all the crap is removed. If the customer is selling the house or just freshening up the garden with new plants etc it'll make a huge difference to the appearance.
Because it's knackered ...see above. lt always look 5hit.
You can't polish a turd.