The bolt holds the noseleg to the engine frame and is in a very tight spot.
We do not have to use the bolt again, but it can not be cut / drilled / welded without taking the engine off.
I have tried Plus Gas with no effect, so Mouse Milk unlikely to help. It is so awkward to get at, that heating / cooling will be very tricky.
I have been advised to use a tapered fork / rod end splitter to lift the bolt out. Will try that next if I can get my hands on one.