I assume that we're talking about normal regular cleaning of commercial or industrial properties, in which case this is outside the scope of CIS - see here for further details.
It's only if the cleaning was carried out in the course of construction work that they would need to register as a contractor (although presumably, if they were already doing this kind of work, they would currently be registered as a subcontractor).
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Pearce & Co - Chartered Accountant and Chartered Tax Adviser