The lower shelf supports came out with a hammer and screwdriver but I needed a crowbar to lever out the top wooden block (to make room for one upright of the ancient Dexion shelving I want to move in) and as it finally came away the roof lintel dropped 2cm. Ah, so that was why it had been so hard to get out! Not sure my Dexion shelves will fit in now - it was a close-measured thing - but if they do, at least they'll help hold the roof up.
Then I cleaned the kitchen. It's now about as deep-cleaned as I'm capable of and will be demolished before anything like that happens again.