NatWest have lost my Icesave Reinstatement Cert - what do I do?!

I was a happy Icesave ISA customer with many years worth of ISA contributions until... they went bust!

The FSCS compensation scheme went very smoothly and I was able to transfer the entire balance into my NatWest e-Savings account. I soon after received my Icesave Reinstatement Certificate which would allow me to transfer this money into another ISA before 5th April (since extended to 5th Oct) and keep its tax free status.

At the end of February I opened a NatWest e-ISA, since it had now started to allow transfers in and was offering a good rate of interest.

6th March - I went to my NatWest branch with my completed transfer form, Icesave Reinstatement Certificate and photo ID. I was then assured that it would be sent by internal mail to the appropriate e-ISA department and I would then receive confirmation of receipt within a couple of days. (The fact that the transfer form did not actually show that the money was now in my e-Savings account seemed OK with the bank staff - I was told that the e-ISA people would contact me directly for this information!?)

Unfortunately that is as far as the transfer process got! Yes - nowhere!! And no-one has any idea where the documents (and importantly my Reinstatement Cert) have gone!! The money only had to go from 1 NatWest account to another - what could go wrong?!

I had tried to contact them by phone several times, received a letter stating that the matter was being 'investigated' and have today (8/4/2009 - 34 days later) received the final nail in the coffin:

Since I wrote to you on the 31st March 2009, I have now fully investigated your complaint.

I write to advise that we have unable to locate the paperwork you handed into the branch on the 6th March 2009. We have been in contact with our Central Tax Unit and they also have no record of your request to transfer your Ice save ISA balance to your new E ISA Account. They have advised that the only option available to you would be for you to advise H M Customs & Revenue, providing as much proof as possible of the Ice save Account, copy statements and any letters that you may have. ....

(Exactly as written - typos/mistakes courtesy of NatWest)

They want me to do what?! Whose fault is it?! The last time I tried to contact HMRC over a relatively simple matter was not a pleasant experience and I don't plan on revisiting that dark place if I can help it!

I have been a customer of NatWest for many many years but sadly now feel I cannot trust them (alas, this is not the only thing that has gone wrong in recent months). If this was a lesser amount of money I would consider ditching the tax free status for a quiet life - but it's not. Many years of compounded ISA contributions does add up!

My serious regret is not coming away from the bank in that initial instant with the name of the person I dealt with or some kind of written receipt - despite my partner (who was with me at the time) requesting something written/stamped - I was not pushy enough and let this slip. Unfortunately the repeated verbal assurances from the bank clerk was enough to convince me. Lesson learned! However, I'm pretty sure I could pick them out of an identity parade!

So, how should I proceed? Any ideas most welcome. Thanks.