Rich
You are taxed on your profit for the year.
You need receipts for petrol, equipment, ladders you buy any thing that is for the window cleaning bussines. This includes car running costs.
So if you have made £10,000.00 in a year and your bussines expenses were £4,000.00.
That would leave £6,000.00 then take off your personal allowance single or married mans.
A married mans was around about £4,500.00 at one time so you would deduct that from your £6,000.00 leaving you £1,500.00 that you would be taxed on.
This is roughly how it works.
Roy