a month beforehand unless you are upgrading when you renew use the options team 0800800030 - they are uk based and will give good deal
From personal experience, they aren't as easy to haggle with as they used to be. I had to request a MAC code. They then contacted me a few days later giving me a second number to call to discuss things. The person I spoke to then finally made me an offer.
When I phoned 0800 800 030, the system gave me two options - business or cancellations. You need to select cancellations.