Of course, accurate figures as soon as possible are always welcome. They're not always possible though. Like I said, 15 minutes is a long time in this game and anything can happen. The figure you are given first might very well be accurate to the minute but by the time you get to the hold and spin a few times, the wind could have changed, there may have been a go around or two, there could be an infringer, an emergency, someone in the hold above you or in another hold could shout "pan pan"....anything could happen.
As was mentioned before, even something as simple as another controller taking the reins could increase the delay by a few minutes because, believe it or not, some may not be quite as good as others.
I haven't been on a fam flight into Heathrow in a while (one in the pipeline though) so I'm not quite familiar with how you plan your fuel but do you plan it that tight? I know 6 minutes can mean the difference between diverting or not, especially if you have been holding for a considerable time but 6 minutes on top of an expected 15 in the grand scheme of things isn't entirely unreasonable.
I agree entirely that we should do our best to keep the figures accurate and that the figures of recent haven't always been as accurate as we would like. However, I resent being accused of lying.