You will need to contact them, they will not contact you. You will go onto a monthly rolling contract and any discounts you had will stop.
If you contact Live Chat they will tell you when your contract is finished.
If you are going to renew contact the options Team on 0800.800.030 and try and get a deal from them. Don't accept the first price they offer you. Make Privacy & caller display part of the deal you want.
As GG has said contact the options team I did last Thursday.
I negotiated unlimited broadband, anytime calls and free caller display for a year, for 65pence a month more than I was paying for out of contract 10Gb broadband and anytime calls. I have LR saver till October.
I thought that was a reasonable offer others may not.