But i am intrigued as to how this system works if the tills work as i assume, all cash goes into till. How do you know the £100 difference is for a specific account when surely all paperwork states that £570 has been paid. Surely in the systems eyes that account was credited £570 and the system has no idea how much cash was actually physically accepted at that time, which is the responsibility of the cashier and thus the human error is derived.
		
		
	 
Our tills work like this:
A 'real-time' cash transaction.  You give the cashier £470 and a credit slip - this is our check.  The cashier counts the cash and sticks it in the till.  THey the enter a 'cash credit transaction' on their machine to tell the till they've put £470 in. In error, they tell it £570 was put in.  Nothing happens now until either the till is balanced or the 'credit' slip is processed that night.  W'll assume the latter.
All the 'real-time' work gets sent off sealed in an orange bag to a clearing house called iPSL - I use bradford but there are several.  The credit slip is put, with the rest of the days work, into a big machine that reads it.  It then checks for a corresponding transaction in the account - paid in at branch 40-xx-xx, into s/c 40-12-34 a/c 12345678 for £470.  It doesn't find this and kicks the voucher out which is duly sent back to branch as 'voucher received, amount not posted' exception.  The system ends it's run and a second exception is created as £570 transaction into 40-12-34 12345678 does not have a corresponding credit in the days work, thus a 'amount posted, voucher not received' is created and returned to branch.
A nice red & grey bag arrives back in the morning with these two bits of paper in and the cashiers take a look.  The image of the credit slip shows £470 but £570 has been posted to the account by mistake.  The cashier then reverses the £570 and puts through the appropriate transaction of £470.  'Dummy' credits and debits are created to keep the machine happy that night.
Sounds complex and it is, but it helps remove counter errors like this.  Just to let you all know, you're just as likely to have something debit twice or for to much and then the it becomes more obvious that the system is there for the protection of the customer. 
Tills are mostly 'pooled' anyway now so they can be shared between many people on any one day.  Gone are the aysof having your own I'm afraid.