Locks that work

Hey all, I am moving to London next weekend and just wanted some advice about using my fixed gear around London. I've ridden my bike previously in Birmingham and around Aylesbury/Milton Keynes, where I have been fine locking (for short amounts of time) with a On-Guard Pitbull mini lock and cable. I'm a keen follower of the Bike Locks that work and How not to lock your bike threads, so I know that this will not be enough when locking in London.

I will be living in Stratford (either locking bike in secure garden or leaving it inside) and riding it to the new Amazon offices in Holborn Viaduct during the week. My offices are supposed to have bike locking facilities as part of the car park, which I assume is underground.

So my question is - am I better off ordering a full sized U lock to use in conjunction with my mini U lock, or a proper big chain + lock that I leave at work most of the time (also in conjunction with the mini u lock)?

Balls sorry, missed the correct thread for this! Anyone got any good 1st hand advice for the best route to go for this?