We had the same recently.
Went into LLoyds, and they wanted to see original death certificates, the will, grant of probate, passports for the executors, inside leg measurements etc, etc.
Walked down the road to Nationwide, and they basically said "Yep there is £40K in the accounts and who do you want the cheque made payable to?"
Also found an old Prudential life assurance policy - taken out in 1929. It was basically a 1d (one old penny) a week policy to pay burial expenses up to £5 if the child died before the age of 5, and £10 if the child died before the age of 10. It didn't say what happened after the age of 10. The insured died at almost 91. We found the policy in the biscuit tin which stored all the important documents, and decided to ring the Pru before shredding it. Turns out the policy was still active, and now worth £1300! The lady at the Pru said they have a special vault full of thousands of the things, but since most people have lost/thrown away the policy document the money goes unpaid.