Assuming that you are a first time buyer remember that the seven year countdown commences from the drawdawn date
In addition in order to answer your query I am making the following assumptions:
Interest Rate 3%
Mortgage 250,000
You are buying this property as a single person
Amount borrowed in April 15,000 @ 3% interest to dec = 300
If this is used you will only get interest relief of 300 @ 20% = 60 and the seven year ends on 2011
However if you wait until Jan 2006 as your first repaymment date
Interest @ 3% 7,500 - Max for a single person is 4,000 @ 20% = 800 (8,000 for a couple @ 20%)
You can claim this 800 in interest relief for seven years i.e till 2012.
as against the 2540 @ 20% = 508 (max allowed after this date)
Hope this makes sense.