Any suggestions on how best to repair this? Particularly in keeping a good edge at the corner to meet the wood at the bottom?
For deep stuff like that I'd be half tempted to get the majority of it filled with bonding plaster (but keep the bonding below the level of the finished surface) and then make tidy with a fine filler like easi fill.
@Nef started
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Any suggestions on how best to repair this? Particularly in keeping a good edge at the corner to meet the wood at the bottom?