On the back...take off your pure water technology as it doesn't tell anyone travelling behind you what you do. Put on MBS Window Cleaning. Any stickers from APWC/FED/yellow pages that sort of thing could go on there too. My opinion, but I wouldn't waste valuable signage money putting "pure water technology" on the van as this can be discussed when quoting. 
Would also have the contact numbers bigger than they are.
I disagre with Helen on the pure water technology part, as this looks professional, anyone that sees window cleaning with it can put to and 2 together or ask and it is the asking or calling is the point of it in the first place, please take it from me, I have been doing marketing for years, I offer a rain guarentee and it is on my van and no body knows what it means people ask me all the time and call to ask aswell, this is called clever marketing.
One thing I would do is change that email address as it looks unprofessional and for the cost of about £20 it could say
quotes@mbswindowcleaning.co.uk or something simalar.
I am guessing you do not have a website, not going into that, you now where I am if you need me.
Back to the van, use more that one colour as it will break it up or it will all mold together and confuse people, use bullit points on the back stating you services etc. there are a few other bits but I have to go for a mo.