As long as the property has been completed by Dec 31st then the Section 23 relief is not affected - but if you don't have a tenant in place before end of 2006, then you cannot claim any relief in 2006. Your tax relief would only commence in 2007 in that case.
But the reduction to 75% of tax relief does not affect you in relation to Dec 31st as long as the building is completed (and maybe even if its only substantially completed).