Partition elimination and date variables
Got an answer from Pivotal - I should have realised this, but parameterised queries are planned in ignorance of the parameter value, and partition elimination is part of the query plan, so we can't use variables for partition elimination.

I am re-tooling our partition elimination to use views that are dynamically constructed with an IN list of dates. This specific query is now just looking at the overflow partition as that's a lot simpler.