I would insist on an explanation from NIB.
The Oct. .5 % reduction was applied to my NIB variable mortgage on 27/10 and my repayment from Nov. was reduced.
The Nov. .5% reduction was applied on 01/12 and my projected repayment for the end of Dec. has again reduced.
They have said they will pass on the .75% Dec. reduction, if they do I would expect it to be applied early Jan. and my repayment at the end of Jan. to be reduced again.
At this stage you should have received the first two letters informing you of the reduction and your new repayment.