One piece of kit you definitely don't want to skimp on is the lock.

The bad news is there is no lock in the world that will defeat someone who (a) knows what they are doing and (b) really, really wants your bike.

The good news is that locks generally don't have to do this. Usually what's required is to make your bike sufficiently hard to steal that they'll go for an easier target.

The job of a decent lock, then, is to make it awkward, noisy and time-consuming to steal your bike.

The highest standard of lock is known as Sold Secure Gold. This means it successfully survived a sustained attack using all tools commonly used by bike thieves, including both bolt-croppers and a portable angle-grinder, for a full five minutes. There are vanishingly few situations where a thief will be willing to make that much noise for that length of time.

The bad news is that secure means heavy. Cable-locks are nice and light but even the toughest of them can be cut using extremely small croppers in less than a second. The only Sold Secure Gold locks are thick chains with heavy padlocks, and heavy D-locks.

My own choice is the Abus Granit X-plus 54. 13mm square hardened steel shackle, shaped to resist twisting attacks, and a double-locking system so that a thief would have to cut through the shackle on both sides in order to remove it.