I will try and explain.
You have bought ÂŁ1000 of stock that you have on the van.
You use ÂŁ500 of your stock on a job and charge it out at ÂŁ1000. So you have made ÂŁ500 of profit.
You also charge the customer ÂŁ500 in labour. This is also profit.
So you have made ÂŁ1000 profit. This will be taxed. So 20% is ÂŁ200 tax to put aside.
But..........
Now this is where you are best using an accountant, or are very good at tax law.
During the financial year you will have business expenses,like van depreciation, tool replacement, stationary, fuel, room in your house for bookkeeping, bank expenses , accountancy fees. and the main one you mentioned, material costs. All these are expenses that do not attract tax.
At the end of the year, you will have an ammount of stock left over in you van or garage. This is expected, but will be taxable. I usually just say ÂŁ500. If you are a 20% tax payer p, that's ÂŁ100.
But as other posts, I would recommend paying an accountant. They should always reduce your tax bill by at least thier charge.
Also if you want to borrow money from lenders in the future. You will need certified accounts going back three years. So best to start now.
Good luck.