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Doesnt need to be portable- really just for locking a bike in a shed- even threading steel weight training weights through as well (or attaching using another cable onto).
if its for a shed try this
http://www.torc-anchors.com/products.php?cat=18
they have other stuff for garages etc
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Go for the motorbike stuff:
The Oxford XL Hardcore and Oxford Monster came top, both £50 and stocked all over the place. Fit a ground anchor too and you're set.
put bingo in the shed!
look at motorbike locks!!
Motorcycle locks - d locks are best followed by chain and padlock with cable locks a poor third. £50 is the absolute minimum for a decent lock and be prepared to pay more.
Abus magnum are good as are the top of the range oxford stuff
Oxford- check. Off down to my local motorbike store. ta guys.