Is there any record anywhere on the license indicating the 5-yearly payment was made i.e. do the CAA send you a receipt, or can the FE check it online?
Yes - a JAR 5 year licence should state somewhere (top of the 3rd page) when the latest you can renew the licence is, ie the expiry date. OTOH, the first few may not have had this bit, merely an issue date! The 5 year payment buys you a new licence with new issue and expiry dates.