In theory a simple, but not always easy, question to answer. The decision in R (on the application of the London Borough of Southwark) -v- London Fire and Emergency Planning Authority  EWHC 1701 (Admin) sheds a little more light on the topic.
The background was that in 2009 there was a fire at a block of Council flats in Southwark, leading to the death of six residents and the injury of others. There followed a thorough investigation and inquest, the possible outcome of which was that the Council might face prosecution by the London Fire and Emergency Planning Authority (“Fire Authority”). The Council argued that the bringing of any prosecution against the Council should not be considered by the Fire Authority, but instead passed to the Health and Safety Executive. This was because the Fire Authority had a conflict of interests by virtue of a number of factors: its fire brigade had attended the fire; were criticised over their handling of the fire; and had provided training to Southwark Council’s officers on the carrying out fire risk assessments for Council housing blocks in the Borough. The Council argued that the conflict would render any decision by the Fire Authority to prosecute tainted with bias.
The Fire Authority denied that in deciding whether to prosecute the Council it had a conflict of interests. It wrote to the Council along those lines on 10 July 2015. In then later made a decision not to alter its earlier decision. The Council argued that this gave rise to two decisions that the Court had to consider, namely, the decision of 10 July 2015 and the later decision not to alter its earlier decision. The Court rejected the Council’s argument and concluded that …”there is but one decision that requires to be assessed. Thus, the original decision continues to operate until it is brought into effect or reversed. In the absence of new facts which might justify a change of approach, a second decision is not made simply as a consequence of a refusal to change the first one. Were it otherwise, the time limit for judicial review of any decision could be restarted following a refusal request to change it ”.
Incidentally, as to whether there was a conflict of interests, the Court found in favour of the Fire Authority holding that there was neither bias nor the appearance of bias in deciding whether not to prosecute the Council.