It's really remarkably easy, you know.
As mentioned in the div thread, I locked up my bike yesterday morning then lost the key. I can only possibly have dropped it within a very small area, but despite thorough searching, I didn't find it.
This morning, I asked around at various places to see if it had been handed in. No joy. Then phoned a local locksmith to see if they could help. Sure, they could - but they very kindly warned me it would cost me in excess of £100.
So, on to plan C...
I got a set of bolt cutters from Screwfix - went for the mid-range ones, thinking the low-end might not be powerful enough, but also didn't want to spend too much. Even so, they're only officially rated to cut up to 10mm and the shackle of my lock is 13mm. Some internet research suggested that I'd struggle to cut through the hardened steel with these, but I found a site that recommended using
a hacksaw fitted with a carbide grit rod saw blade (as used for tile cutting). So I also picked up one of these from B&Q.
I tried the saw first - thought it would make me look less like a hardened criminal than carrying round a bloody great set of bolt cutters (they're really quite hefty). Did the trick nicely. Quite slow going, mind - took me about half an hour to cut all the way through, including breaks to rest my hand (it's quite tiring work), and I had to swap the blade round about 3/4 of the way through because it was getting blunt. Probably could have done with getting a spare blade or two. I don't know why, but the last few mm seemed to be the hardest to cut through. Fortunately, I only needed to cut once - that allowed enough wiggle room to release the bike.
I'm slightly disappointed that no one challenged me while I was doing it. Perhaps I look too honest. Or perhaps it was the fact that I was being brazen enough to do it in broad daylight in a busy place - plenty of people walked past, and only a couple even bothered to give me a slightly suspicious look.
When I got home, I thought I'd have a bit of fun and see how I got on with the bolt cutters... Blimey. It was surprisingly easy to cut through the shackle. I used my foot to put my full weight through the handle but even so, I'm not all that heavy!
So, there you go. I'm not sharing this as an instructional guide to thieves - I'm sure they know all this already. But it is worth knowing should you ever find yourself in the same situation as me.
Also a stark reminder that however secure you think your bike lock is, there's really not a lot that will stop a determined, well-armed, knowledgeable thief.