Surely your mate can knock something up in his shed to sort this out.
No that needs a different approach to sort out that problem. No amount of chemicals,scrubbing or ice blasting( 
) will fix that.
There will come a time in your life when you will get fed up with talking about stuff that you know nothing about. Do you ever get fed up with talking nonsense? Try and bear in mind people may be daft and naive enough to listen to your rambling
Kelvin.....You obviously have not reached that time yet then...have you?
Look at the picture I don't see a traditional damp coarse.
I do see "new" hardcore that might have breached the damp coarse.
The area is green with algae ...this tells me it is continuously wet ...possibly from the lack of damp coarse or other?
Is the area north facing..in the shade of trees/building ,are there dripping gutters,leaking overflows?
You need to address the problem that is causing the efflorescence before cleaning the area up.The clean up is easy.
Do the job properly the customer will remember you for that.
Ice blasting will remove the soiling ...very expensive. B&Q for some brick acid will get the same results for a tenner.
That or my mate (in his shed) makes a very good salt neutraliser