It may be worth you investigating the cost of subscribing to an organisation called Dunn and Bradstreet. If you are a member it is possible to report various companies that are poor/bad/default payers, and receive history on a particular company, if they hold it.
Another option is to undertake a credit/paying score for most companies or U.K busniess. Try,
www.duport.co.uk, they are reasonably priced and quick.
One option I used in the past, especially if the contract was in excess of £2500 per month was to factor the invoice.(Fundamentally sell the invoice to a bank or finance company. You get paid every month, less a percentage and they chase the bad debt, if it happens) Lloyds TSB are good.
Other than that put in your T & C's a two month clause. Not paid within 56 days of first invoice, goodbye, farewell, adios and see you in court. The trick is to invoice on day one of the contract for payment within14 days after completion of first month. It comes down to how you set up the contract and the T & C's you agreed with the client.
If you are unsure about the client, Paper trail everything. Carry out a monthyly cleaning audit, with the client. Get their representative to sign the cleaning off every month. If it goes to court you have proof that you fulfiilled the T & C's of the contract between you. (It's called covering the area between the small of you back and the tops of your thighs)
A small cleaning contract can soon become a big deficit on the old cash flow.
Nils Desperandum!