Please do check with Citizen Advisory Bureau or a Solicitor
Home office like to confuse people becaus ethey are confused themselves. There policies are very vague and always leave confusion in the, You need to contact Immigration solicitor or Citizens Advisory centre. here are the rules as I know but there will be peole with different understanding.
The date your Visa starts is the day it is stamped. In your case it is 15 December 2005. If you pass 15 december 2007, you are not staying legally in the country.
My visa was issued on 15/11/01 to 15/11/02(They used to give one years Visa at that time). I entered the country on 23/12/2001.
I applied on November 14(I think). My 1 year was not completed at that time. I was married in 1985 so it wasn't important that I prove my marriage still exist.
Anyways, you should apply between December 12 and 15, 2007 and you should be fine. They have 28 days that you can apply before your visa expires. and as you enterd the country on Jan 12, you will fulfill their requirements.
Please do check with Citizen Advisory Bureau or a Solicitor