I'd been expecting a struggle, but it wasn't hard. I made up some paint stripper (the joint was well-painted-over) [1] and gave it a few whacks with a cold chisel, and it came apart no bother.
[1] Peter's Paint Stripper: mix 3 tablespoons caustic soda with three times as much water - warning, this will get hot. Allow to cool - I use a jar in a pot of cold water in the sink, so any spills are easily washed away and it cools quickly.
Mix a teaspoon of cornflour in about 3/4 pint water in a jar with a sealing lid, stir thoroughly, and add the caustic soda mixture. It will go very gloopy. The gloopyness helps to keep it in place in use - paint on the paint, wait 30 minutes or so, then wash off.
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