If inc mortg on inv property is extra interest tax deductible?

Been reading here about the wisdom of maximising investment property mortgage and minimising principal private residence mortgage.

BUT....

I'm in a similar situation to this post

http://www.askaboutmoney.com/showthread.php?p=187914

So do the revenue allow you to use the additional interest payments following remortgaging against your tax liability????

Very important question for me.
Thanks in advance.