Ian
Well done, it is a decision that in a few years you will be wondering why you never done it sooner.
Now you have done the hardest part by resigning, now comes the easy bit. Building up a sucessful business.
Before you start a part time job, check with the tax credits office if you are elligable, also apply for council tax and housing benefit.
Here is a list of what you can also do
1 place an advert in all the free search engines on the web, the likes of free index are the ones you need to look for.
2 Tell everyone that you are now working full time as a window cleaner and you need their help, if they can actively try to get you work you would be grateful.
3 Approach all the local shops and businesses in your area to see if you can place a leaflet in their window (free of charge) advertising your business, you might also pick up some of these as customers.
4 Knock doors every day for the next 8 weeks, this will soon build up a round.
5 Put at least 15% of your taking in a safe place for emergences.
6 Don't let it stress you, remember you are running a business not the business is running you.