Well, I'm happy to report that the snake lock is off - I shall be back in the saddle to work next week! Hurrah.
A tip for any Brits out there (it may also work in the States) - the answer to "how do I get the lock off my bike" is...
...ask a nice fireman to help you!
I'm fortunate in that there is a Fire Station about 5 minutes' walk from where I work. I asked if they would help me - and they replied... sure, no problem. Top blokes!
Incidentally, the bolt clippers didn't work. They only cut through the plastic sheathing; it took a hacksaw to go through the cable itself.
Anyhow - thanks for all your help; much appreciated.
Matt