This is ringing lots of alarm bells.
Why would anyone hand over £1000 up front, then £51 per week for life to advertise someone else's business (on your van by the way) and for you to have the privilege of doing their work?
Any established window cleaner will be able to see right through this scam, which is why they are looking for those looking to start up. What happens if you sign up, then want out? With a good credit rating being a necessity, they will be looking for you to keep paying whether you keep doing the work or not.
To anyone wanting to start up ... Canvass, canvass, canvass. There is plenty of work out there, just get out there and find it. If you sign up for this offer you will almost certainly regret it.
Best wishes,
John.