Having taken it for a ride down the road to check the minimum required to make it rideable, I did the minimum required to make it rideable:
* repaired the slow puncture in the front tube
* stole one of the limit screws from the rear mech for the front mech and set them both to keep it in big ring and about the third highest gear at the back
* added a middle ring at the front (bolts were too long without and took the number of chainring bolts up from two to five
* disassembled and correctly re-assembled the rear canti that was jammed against the rim
* tightened the very loose headset
* lubed the chain
* added grips (probably not essential, but more comfortable)
Then took it for a test ride to the pub (about 8 miles there and a little longer back to get off the roads for awhile).
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Given it still needs a lot doing, not least the wheel bearings servicing, I was surprised how nicely it rode. It goes to show how much difference a frame makes - I feel this one suits me, so I definitely plan to replace my current all-purpose bike with this.