3 monile Lowell payment req

missprice

Lowell have sent me a letter asking for payment for an Three mobile bill.
on the reverse is a statement

opening bal 1/10/2008 £35.23
total payments recvd £0.00
closing bal 31/05/2013 £35.23

now I remember the amount and am quite sure I paid it in 2008. but nearly 5 years later I cant prove it.
usually for these things (closing of contracts etc I keep for 3 years and then think ok they wont chase me now so shred it.
So now the proof of payment is gone (I checked) what can I do?
also changed bank in 2011 so cant even ask bank for previous statements.

if I just send a prove it letter will that do?
should I just pay it again and keep proof for 8 years although that would stick in me craw:)