STD Code mistake
The list of numbers in the diary from 1976 have the STD code 01946 which wasn't introduced until the mid 1990's.
shareAh but 01 946 1234 would have been a valid London phone number from 1958 (when STD codes were introduced) until 1990 (when 01 was split into 071 and 081).
However there is a catch: 01 946 xxxx was never used: https://en.wikipedia.org/wiki/Fictitious_telephone_number lists it as a fictitious code to be used for TV and film.
Update: In episode 2, we see a close-up of the list as a detective is on the phone to one of the people in the list. The original number 01 946 xxxx has been updated to the present form 020 7946 xxxx. That makes it clear that it really was a London number (although a fictitious one for the TV series).
The london codification was modified briefly in the 70's when the Hennesley-Rampart system was being tested.
This required all call recipients to face the Shoreditch Telephone mast when answering calls and hold-in their left hands- a Jerusalem Artichoke. The system was withdrawn for cost reasons and after multiple protests from the West London Church of the Wandering Herdsmen.
A fascinating account of these events can be read in Crotchley-Vanguard's 1982 novel-"my life with Albert".