There seems to be some confusion here, let me clear it up for you.
You send an invoice for £100 + VAT @ 17.5% Total invoice due £117.50.
Now you do this on every invoice irrespective of the amount, i.e £5 + VAT, £50 + VAT etc....
Now you keep your records EXACT with details exactly how much VAT you have collected from your invoice.
So using these figures as follows:
Jan invoiced £10,000 + VAT = £11,750 (£1,750 = VAT proportion)
Feb invoiced £9000 + VAT = £10,575 ( £1,575 = VAT proportion)
March invoiced £14,000 + VAT = £16,450 ( £2,450 VAT proportion)
So total amount of VAT collected in this quarter is £5,775. This is the amount you HAVE to send to the VAT office less any VAT you have paid yourself on any purchases.
You claim back ALL VAT on purchases from suppliers, rent, petrol, stationery, anything that you have paid VAT on.
So lets say over the same 3 month quarter you have paid out £2000 VAT on your own purchases you then only send the VAT man the balance:
VAT Invoiced £5,775
VAT spent £2000
Total Due to VAT £3,775
Hope this clears it up for you?