IHTM10070 | Bank and building society accounts (box 52) what accounts are included?
From HM Revenue & Customs · Inheritance Tax Manual
The following should be included in box 52 of the IHT400
current, deposit, high interest, fixed interest, term, bond and money market accounts with a bank, building society, mutual, friendly or co-operative society
accounts with supermarkets or insurance companies
cash in an Individual Savings Account or other tax exempt savings accounts.
Details of the investments and figures for the capital and interest owed, but not paid, up to the date of death should be shown in box 1 on form IHT406.