Harlesden might not be the most attractive area of London but, when it comes to geocaching, it's like a white spot on the map and deserves a few quick grab caches.
You are looking for a tube sized, small, magnetic cache. You will have to bring your own pen to sign the log sheet. It's a quick grab cache. The area is farily muggled during rush hour but otherwise there shouldnt be any problems accessing it 24/7.