BB, that navigation error, whilst a gross error, was not unusual at that time. They would have been out of rapid fixing cover, Gee/G-H , and reliant on a radio compass and radar - H2S Mk4.
It is possible that emcon rules prohibited use of the H2S radiating towards the IGB.
The radio compass would not have had an auto-sense aerial and it is possible, like Lady be Good, that they may have had a bad bearing.
If we knew what the weather conditions had been like, such as 8/8 cover below and no visible astro bodies, then we might know more.