Not sure why Josh Piterman is returning to Australia. It can't be something that's super urgent if he's staying on for another nine days. It's a shame, cause I like his Valjean (much more than I liked his Phantom): his is an emotional, compassionate take on the character and I like the slight melodic liberties he takes with the recitative.
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Donnelly will make a fine replacement however. He was a fantastic Valjean cover back in 2010 and I wasn't surprised he got to play the role "for real" both in London and in the UK tour.
London currently has three Valjean understudies: Christopher Jacobsen (Combeferre and long-term Valjean cover/alternate), Brad Barnley (Factory Foreman) and Matthew Dale (swing). Presumably either Jacobsen will take over the lead, or the three actors will share Valjean duties, between the time Piterman departs on the 21st and Donnelly joins on the 31st.