The standard for accountants valuing a commercial business is generally that any business is worth 2 x THE ANNUAL PROFIT.
However, commercial and domestic are totally different beasts, you would need to look deeper into the books of this business to see what their costs are, also are they on fixed term contracts, tenders, open-ended, have they had any problems with the customers, or vise-versa.
A £30K business is most likely a one man operation, how long have they been established, what is their reputation like, why are they selling?
If you have contract cleaning business now, you're probably better off just buying the gear you need and starting out from scratch.