So if I understand this right, you have a wedge-shaped space between the side of the shed and your fence, which the scrotes used to gain access? If the large end of the wedge faces out into your garden rather than into the corner, then why not infll the gap by building a small wedge-shaped wooden store rather than just waste the space? That would then mean the exterior wall of your bike shed would be behind the infill construction and no longer be accessible. The storage space created might not seem very useful, but even so I daresay you could manage to use it to keep a few tall skinny items in like long-handled gardening tools, brooms etc.