I'm a bit confused by your post as you haven't said whether the little bit extra is extra money before tax and deductions, or afterwards. So to cover all bases...
You should have a payslip - have a look at this. In the gross payment (before tax) payments - often listed on left of the slip - does it list any additional payments underneath your salary such as holiday or overtime - if so, this could be holiday entitlement (holiday you have accrued but not taken) which employers are legally bound to pay out with final payment.
If no seperate payments are listed, look at the total gross amount and compare this to your previous payslip. If its a higher amount, then it could still be holiday entitlement which the employer just hasn't split up into the different portions.
Also, have a think about when you left in the month in relation to when payroll has been run and when the payroll cut off day is - it could be that your last day would normally be paid in a later payroll run but for reasons of simplicity, your employer has added the extra days to the normal lump sum.
If the gross amount of pay is the same, it will be due to less tax and NI deductions - your employer won't be able to help you with this as payroll systems dictate the amount of tax and NI deducted and not the employer.
As such, you will need to phone your local tax office - to find out the phone number, if you don't know it, look at your P60 or your P45 - this will list your employers PAYE reference code; then visit
http://www.hmrc.gov.uk/local/individuals/index .htmand enter the first three digits - this will come up with the contact details.
That said, if what you're implying by your post is that you've been included in a pay run which you shouldn't have been because you left then DON'T spend it. They will come for it at so