Do you have any records / statements etc yourself? If not you can request what Egg (Citibank or Prudential now) have on you by requesting your personal data from them. (Data protection Act 1998 section 7) You need to make a Data Subject Access Request but they can charge £10 for this.
Write Citibank and Prudential a letter requesting a DSAR and include your name, address/previous addresses and date of birth so they can find your record. Include a £10 cheque with each and hopefully one will answer with info. The other will return the cheque. Good luck and DON'T use a claims firm. they will do the above and sting you for 25% of any compensation received.