How long will it take? Price for the time...
at what rate £150 per day
Take full day if not a bit more
If it's going to take you a day or more I'd at least double that to £300.
Have you explained to the customer the full details of the service you are providing? Doing so will make it more likely that they will accept the quote as they can see the benefits more.
Just seen the picture on you're original post. It's the Holiday Inn mate - I'd be quoting at least 500 notes for that. In your quote play massively on the health and safety factor and how you're fully health and safety compliant.
I'd also give them a couple of options to choose from, eg:
Option 1: Cleaning of all internal guttering and clearance of any blocked down pipes.
Option 2: All of the above + cleaning of external guttering.
I'd also enquire about the window cleaning for the site. They may already have one but what if he is trade and can't do all the glass or if they are just not happy with his service. A one off job could then become a regular just by asking the question. We all leave money in the table by forgetting to ask these questions from time to time.
Good luck with it mate.