I am assuming an internal investigation may have taken place at the time and Dr Gibbs will be cross examined in regards to whatever the findings were.
Guessing as he was overseeing the childs care he would of been involved in the process?
Pure speculation, but if it was deemed accidental/whatever the nhs equivalent is, that is the area reasonable doubt could be placed. Would explain why bm has deferred his cross, it is just not the place he needs to cast his argument. Utter guesswork on my part here for nhs procedure/process