Clean master,
If you are paying them £6.50 then your true actual cost will be closer to £8.50.
You need to allow for your NI contribution and holiday pay. There are other overheads including vehicles, fuel, uniform, advertising, equipment (vacuums, brushes, etc)
You're probably still making a reasonable profit, just not as much as you think you are!
As a general rule we will work on 40% profit on jobs like this, our minimum hourly rate is £20 + vat, however we have a created a spreadsheet where we load in number of rooms, bedrooms, stairs, windows, doors, kitchen, bathrooms, etc etc and it gives us a magical figure for what the price is. It's not an exact science but it gives us a good feel, we use the same type of calculations for build cleans.