Thank you Lillie for your reply and suggestion. Unfortunately that did not have the desired effect.
I tried selecting using the system track as you suggested and the wanted bars were selected. When I tried transposing using ‘Write > Transpose’, the “Transpose key signatures” option was then available and not greyed out.
However, when I applied the transposition, the entire song transposed including the new key signature and not just the selected bars.
I retried and checked and double-checked that only the wanted bars were selected, none of the other bars in the song were selected just the wanted bars. Once again, when applying the transposition the same thing happened.
Clearly I must be missing something here.