Just leave the fork in one bit (loose, but spring in – cap on).
Slide an (8mm?) spanner down the long bit of the allen key. Insert the allen key in the bottom of the fork/bolt. Run the spanner down the allen key so it is resting flush with the bottom of the fork (to minimise ‘rock’ when force is applied). Get a hammer and ‘shock’ the bolt loose by hitting the spanner HARD/quick … works EVERY time - done quite literally dozens of them this way!!
This will work with a ‘ball-end’ allen key, but best with a flat end allen key – the ball end ones don’t have a good a contact area on the flat/standard end.