The Post Office (or Consignia) should be able to help; these five digit codes are used by companies who use "Mailsort" for originating their 'Junk mail', also other firms will show the same code.
This is a method of sending post at a reduced postal rate by having it pre-sorted by computer.
Check the address on your bank statement or credit card statement, it will probably show the same. This is a national system, similar to the postcode.
At least, this is my understanding!

Nigel