By the way mark what do you think I should charge by the sq mtr/ sq ft?
Depends on what system you are using HWE or LM, maintainance cleans or corrective cleans and the amount of work you are getting.
I always offer 10% discount if payment is made in full on completion of job, obviously you add the 10% onto the quote in the first place, these people tend to hold back payment as long as possible.
If you do decide to do work for this company you need to get things sorted regarding invoicing and payment terms cast in stone prior to doing any work.
Mark