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Ah yeah I did think you worked there but wasn't sure. Were they still building when you were there? Do you know who this frame was built for? I may also take this to a local framebuilder to check out the paint flake around the seattube lug. It doesn't creak but looks like it could turn into a crack after time
May also become an excuse to repaint this, theres a few chips and the paint is fairly tired
Harry Hall Cycles is still alive and kicking as far as I'm aware - I worked there for almost 3 years while I was back up north at uni. However, their days of frame building are long gone and I'm pretty certain they wouldn't know where to start with this.
I'd say your options above are the best - rethread with a larger diameter bolt or get a new one machined!