Update, of sorts for those interested. Slipped the fork out, and the inside of the tube is fine. Minor corrosion on the headset, steerer of the fork, all within expectations for a bike of its age. Wet’n’dry on the cracks, not as horrendous as I’d feared.
On advice/warnings/sympathy/scorn all gratefully taken on board, I rode (yes !) to the local aluminum window frame maker, who has promised to drill a couple of small pilot holes (oo-err missus) and tack the thing up. For a couple of quid, I am happy to take a chance. It’ll be relegated to shopping and pub duties, but shall be loved in its dotage.
Once again, thanks for taking time, I’ll stick a pic up when it’s done.
Jon