With only a few butterflies - essentially just change Chris' mind in about 2008/9, where he did believe he had it in him to stay in parliament for another term, but wasn't sure if he actually wanted to. Delay his decision to step down a few months and once Michael Martin's fate becomes clear, he could choose to stick around and stand. Mullin would have been a very strong candidate and it's difficult to see who could have beaten him.
How does 2009-present play out with Mullin in the speaker's chair? If nothing else, some memorable exchanges in parliament may have gone differently. There might be a nice 'soft' TL to be written, in the style of his diaries, about his perspective as Speaker.