Hi Petra
There are many ways to value a company but one of the simple methods I use is 30p in every £1. However this will only give you a ballpark figure. With cleaning companies alot of factors come into play such as what type of contracts are run, the % profit, whether you are buying as a whole or just the good will, liabilities etc.
At the end of the day any company is only worth what someone is willing to pay for it!
Regards
Fox