Subject to seeing the work and the 4700 per month on monthly accounts I would snap your hand off at say 5 x. I unfortunately don't live in Portsmouth, nor do I want to move there. Not that there is anything wrong with Portsmouth!
Blue Monkey - To state that you don't know many people who have 15k in Portsmouth is a little bit blinkered. Everyone who is in window cleaning is not on here.
I would approach contract cleaning companies with this and see if they are interested. 5K a month needs to be sold properly and I would get advice. Go to Business Link and see what they say.
I have sold a larger round than this and I got the money I wanted but I did have an employee who had a way to get the capital.
There may even be a large window cleaning company on here that just browses, rather than posting regularly that may be interested. There are over 20000 members on here and only a handful that post.
Put a letter together and see if you get any interest from commercial & industrial companies.
Good luck
Rob