If you want the all the work done, including the windows under the same business name, you would have to sub-contract the work out...unless I am missunderstanding you. I don't think you will find any w/c who is legally registered and insured who will take money under the table, not that you're implying this.
Sounds like you want to have various tradesmen available in case your clients ask for services you don't necessarily provide, that way you can keep your clients happy, and with you. Is that right? Seems to me that you can still do this without subbing the work out by just telling your client that you have an excellent company who can take care of that for you. Then call the w/c and make the arrangements for him to bid the job.
Clear View, you're over complicating things. The process works something like this:
A company has asked you provide a quote to clean its premises; including the windows?
If it's a BIG job get quotes from three window cleaners. As Mike G says, add a bit more on. It's your administration charge for sorting out a reputable window cleaner for the company.
Add this price onto what you're charging for 'general' cleaning of the premises.
There's your quote.
If you win the contract:
Then the window cleaner turns up at the appointed intervals.
The window cleaner submits an invoice to you.
You pay the window cleaner.
You invoice the company for the general cleaning and window cleaning.
Notes:
If it's not a big job - don't go phoning around asking for quotes. For a small job, just phone one window cleaner. If he seems 'way over the odds'; then try another.
Make sure your window cleaner has public liability insurance. Make sure you get a copy.
Don't go for the cheapest quote - go for the company that you think will deliver the goods.
I know one local cleaning company that has a 'regular' window cleaner for regular and one-off jobs that it sub-contracts to. I'm sure you can find similar if you phone around.