I recently learned the hard way this bit of useful info:
Any cleaning work carried out on a sub-contracting basis on a construction site (including cleaning on completion) i.e builders cleans etc. requires that you have a Construction Industry Subcontractors certificate.
If you don't have one you might not get paid.
They are pretty easy to get, you just need to fill in some forms and give a passport photo.
Just thought I'd share that with you if you didn't already know.
I'm applying for mine now so I can get paid for a cleaning job I did for a shopfittting company on completion of a refurb.