Think you may find that flat surface pretty useless with that machine. I used to have a 15 lt per min machine and even then i always turned to a turbo nozzle as the flat surface cleaner doesn't give great results and found using a turbo nozzle just as quick. That machine has 12lt per min and the surface cleaner is recommended for at least a 14lt per min machine.
Perhaps ok for decking but it wont shift stubborn staining on patios etc.
If using the custys tap i recommend just checking the flow when pricing, I remember doing one job and forgot to checked there tap flow and when i went to use it it flowed like a dripping gutter and the job took forever as the washer was using it quicker then the tap could supply water. Also good idea to check soakaways as if there blocked you end up with a load of standing water.