Horseshoe style locks are generally favoured on continental style roadsters or shoppers (they are also called nurses locks, but more accurately they are immobilisers), whereas in the UK cable locks are the norm - so why not get the best of both worlds? The RS400 horseshoe lock uses an Allen key-bolted clamp to mount on to the seat tube and another similar bolt lets you vary the angle of the horseshoe to suit just about any bike with conventional seat tube geometry.
There's no reason why you can't use this as a standard immobiliser on its own - just turn the key and push the 'trigger' down to slide the toughened steel shackle through the rear wheel then remove the key - but to make the most of it you'll need the cable too. Loop this through a fixed object, through the front wheel and back through the eye on one end of the cable, then push the ball-shaped end into the locking hole on the horseshoe lock. Your bike is now 'double locked'. When not in use, the cable comes with a handy frame mount (optional pannier rack mount included).
A bit more security would have been nice - though the horseshoe rates a 4 out of 6 on Trelock's own security system ('protection against professional thieves') the cable is only rated 2 ('high protection against casual thieves) which seems slightly illogical. However, just the sight of a double locked bike should put off all but the most determined thieves. If weight or high level security is paramount this is clearly not the lock for you but for cycling shoppers, or just anyone who has to make regular short bike stops for whatever reason, this system is well worth looking at.