I will try and explain.
You have bought £1000 of stock that you have on the van.
You use £500 of your stock on a job and charge it out at £1000. So you have made £500 of profit.
You also charge the customer £500 in labour. This is also profit.
So you have made £1000 profit. This will be taxed. So 20% is £200 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 £500. If you are a 20% tax payer p, that's £100.
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.