Paul,
Good thread, interesting to know what people are charging and what customers are paying for this type of service.
Contract cleaning is a growing sideline for us, we have picked up contracts from customers who have generally either been unhappy with they previous contractors, or more usually, they have had their own employed staff and have been unable to retain regular cleaners and just got sick of the hassle of advertising / recruiting constantly. We go in and take the hassle away from them having to sort out absence cover, holidays etc.
Maybe we do work on a good margin, but I have just put your figures into my costing model and I think that at £10/ph your gross margin will be less than 10% and thats only taking into account what you have mentioned, with machinery depreciated over 2 years. I have not taken into account for any supervisory role, admin role, payroll, recuritment time, payroll software etc which would all add on to the wage bill. I have assumed that you are using part-time staff and keeping their wage level under the tax threshold.
At £10.00 / hour I would only expect to be paying staff minimum wage, or close to it, and the customer would supply and maintain all equipment and consumables!
Good luck if you get it, but I think at that rate you will find it a lot of work for very little gain.
I would be looking at pricing this job at around the £12.50 - £13.00 / hr mark. If we got if good, if not, not overly bothered, we are building a good reputation for service, quality and reliability, but people know that comes at a cost.