Hang on, no. It's not working.
I've got a total VAT figure only here, and I need to work out what the cost was before the VAT was added.
You're assuming I've got a figure that's the total INCLUDING VAT. I've only got the VAT total that I've received from my cussies.
Ah! Sorry
If I may say so your book keeping is a bit of a mess.
Surely you price a job, write an invoice (two copies) showing the price and add VAT so the invoice shows both pre and apres VAT totals.
All this working backwards from only one figure is guaranteed to cock up.
I know that's where you are at but for the future ..........................?
Yea, I know. 
I've spent hours working out who's paid me over the last 4 months, and just adding the VAT paid figures up.
I should really have added up the invoices paid before VAT, and then it would have been simpler like wouldn't it...I think?
So, is there no formula for working the VAT backwards to find the cost before VAT then?
Have I got to go back to me books and paying in book to add everything up again before VAT, and then simply mutiply by 15% and 17.5% to double check the VAT received matches the all the paymants received?
Have I made that clear enough do you think? 
Why don't you type the numbers into a spreadsheet... you can do what you like with them then... 
Ey?
Is that some sort of digitalisation scheme for anna log maths then?
It should be dead simple this, really.
?115 inc VAT has ?15 of VAT in it if it the ?100 was charged at 15% VAT, yes?
So, I've now got a sum of ?15 in me hand...cash to make it easier like.
How do I simply work out using a formula what the price was before I charged the VAT, which as we all know was ?100.
It's just the formula i need.