Locating a old trust fund account NatWest

I am currently trying to locate a unit trust fund investment that was originally arranged by my grandad for me to access when I reached 18. This was invested in with NatWest bank in 1996. My grandad passed away some years ago but with hindsight he put my mother’s name on the trust as the second named adult. I have visited my local NatWest branch as instructed and they were unable to help me as they were unable to find any records of such a fund. at this visit, staff took photocopies of both mine and my mother’s identification and all documents we possess relating to it. We were told that the database originally used to setup this fund had long expired and that the system held no record of the fund in question anymore.
Following numerous telephone calls to a number of departments we were promised that this would be investigated and we would receive correspondence of some kind kind in due course. This was back in June 2013 and to date no contact has been received. When this was followed up no-one took responsibility but were instructed to visit the Wood Green branch as this was branch the fund was originally set up at. Upon visiting the branch, yet again no-one was able to assist us.

I was then forwarded to a NatWest representative & I wrote to them. They concluded that the funds had been encashed by my Grandad somewhere between June 1996-September 2002, yet I still have documents of the funds manifestation after 1996 and my Grandad had not stepped back into the country since he set up the unit trust account in 1996 so this would be impossible. My mother and father have purchased and investigated his files both in the U.K. and abroad to track any funds or transactions relating to NatWest during the said period and nothing has been found. In addition, any funds have certainly not been paid to any executor as he did not have an executor long before his death.
I have plenty more information & letters back and forth with NatWest. Something I found extremely odd was that the NatWest representative that I traded letters with back and forth had offered me a monetary gesture of £200 (nowhere near the amount invested by my grandad), Of which I turned down - it was almost an insult to me. My stepdad is going through something similar with NatWest at the moment as his father has just passed away and is trying to locate his father’s bank account from 1965, again NatWest have told him they have no record of the account. Something isn’t right.


Naturally I am frustrated with this whole process as my grandad left me these funds and trusted the bank in good faith that they would be available for me to access when I was 18, I’m 21 now and still no further on in my pursuit to gain access to any details or how they have matured over this period. My grandfather was led to believe that investing in such funds for me, given the choice by NatWest to have them available at 18 years of age would actually manifest. How can you sell funds to customers with a expiry date that you can not fulfil due to not having access nor record of the original details? How is it that they can happily take funds from customers not knowing how on Earth they can access them in the future?

I plan to take this legally but have no idea where to start. Any advice would help me vastly.
Thanks