i use hot pure water and virosol usually for plastics....occasionally ill use CIF cream cleaner too if the plastic is old and has lots of ground in dirt....i use a backpack for the applying the virosol,i just squirt CIF on the brush if needed....
Biological washing powder (any brand) is as good as any, there's no need for pure so if you can then connect straight to the customers tap, the extra pressure makes all the difference.