Hugh
Logistics for such a large area could be a nightmare. Fetching and carrying large quantities of water could be a No No. I would definately clean as much as possible with an LM system. Rinse/extraction would be only a last resort. Noise may be an issue.
You will need everything to be upto date re. PAT testing. If you have a £5k basic products liability insurance it will be totally inadequate, so too may be your Public Liability. If you have any help, do you carry Employers Liability? Check all this out before you go. If you're asked questions, you'll look a fool if you don't know.
Method statements, MSDS, H&S policy etc. etc. may be asked for.
Be prepared.
Safe and happy cleaning
Ken