Hi Blue,
Regardless of how clean this chip shop is, what they are asking you to do is remove the grease, grease only responds well to heat and chems!
Because you do the windows there already, I assume that you know how difficult it is to remove the grease from the inside of the windows, and how dirty it makes your scrim, blade, mop etc.
I also assume that the shop will require wall and floor tiles to be cleaned between 4 & 6 times per year.
My advice would be to spend £300/£400 on a commercial steam machine providing the shop will let you have a years contract, then market your "NEW SERVICE" to similair outlets.
If you decide to follow Matt's advice, ensure that any chemicals you use are foodsafe, and be prepared to really earn your money, cos degreasing is very hard work.
Regards,
Rob