EAT are not guesstimates. They are what us pilots use to plan whether or not we have to divert or can commit to an airfield. Changing an EAT is not really acceptable as the flight may well have committed to LHR and not have fuel to go any where else. I totally agree that RT is not the place to discuss such issues but controllers (and London ATC are the best) shouldn't change EAT's once issued.
For example a crew coming into OCK on a nice day has 35 minutes holding plus fuel for the approach from OCK plus Reserve. Remember that as LHR has 2 RW's Diversion fuel can and may well have been used at this stage, but in this case the crew elect to use it as holding fuel. Pretty much what happens everyday in the holds around LHR. EAT given as 30 mins from hold entry. Every thing fine and dandy. With 10 mins to go ATC put the EAT back by 10 mins. Now the crew have no choice but to make a PAN call. All the crews decisions are based on that original EAT, all the correct procedures have been followed, unfortunately the action of the controller has put the aircraft into a dangerous position. That is something that should be discussed but not maybe on frequency.
On a nice day most aircraft in the holds around LHR are using their diversion fuel to hold. It didn't used to be the case but it is now. The giving of an EAT is vitally important because it means that a crew can commit to an airfield with only one RW, it greatly changes the decisions that a crew can and will make. Changing an EAT can put an aircraft in a very awkward situation. I hope controllers realise just how tight the situation can be these days and the implication that an EAT has to the decision making process.
There are those of us who feel the current fuel policy is just a bit too tight and carry extra fuel. However it is up to the CAA to regulate and they say it is acceptable to leave the hold, on a nice day, to a single RW airport with only enough fuel to make an approach plus Reserve. One day soon a controller will mess it up and change an EAT by 10 to 15 mins and every aircraft in the stack will declare a PAN, followed by MAYDAYs as panic sets in. That will be an interesting day. What will make it even more interesting is when some of the Easy/Ryan guys find that they can't get in Stan or Luton for 45 mins because of all the PAN/MAYDAY diversions. Cue for more and more MAYDAYs.
The min fuel we carry is dictated by the CAA. Carrying any extra is up to an individual Capt but on a nice day there should be no need. The changing of an EAT is serious and could have very very serious implications for all the London airports.