How much old Post Office Savings book balance worth today?

My mother passed away in December. In going through her affairs I found a Post Office Savings book. It had a balance of £1, 1 shilling and 7 pence in 1954.I have sent this to National Savings and Investments and it has replied with a letter saying that the value is now £4.88.I find it quite hard to believe that it has made so little gain after 64 years, do you think this is correct or has it ma...