The British Bankers Association, the Building Societies Association and National Savings and Investments (NS&I), have launched a joint venture,
My Lost Account to replace separate account tracing tools. Use this to find lost current accounts, savings accounts or premium bonds. It's estimated the unclaimed cash in these could be as much as £1 billion, so it's worth it.
To start reclaiming, simply go to the site, select 'search' and you can fill in the details about the account you are looking for. After that it's a waiting game; banks and building societies say they'll respond within three months of the claim, while (NS&I), which provides premium bonds and a few other savings products, says it'll get back to you in one month.
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Found this on the Money Saving Expert website, some people who have had dormant accounts have had quite good results.