I suffer from this myself occasionally. My understanding is that BT runs monitering software from the local exchange. If it detects a problem with your connection (which could be a line problem, a problem with the phone wiring in your house or a router problem) your connection speed will be autmotically dropped to an acceptable safe level. The BT software then monitors your connection for the next 3 days (keeping the safe speed) and if there are no further problems - restores the correct speed.
I am guessing this is why your ISP has asked you to hold off for 4 days. Running the BT ADSL speed test shows the capped speed (line profile -they call it)and the results can be accessed by BT.
Although it hasn't happened to me apparantly the line profile can get stuck and needs to be manually (by BT) occasionally. Just monitor your speed over the next few days. Another idea (which the BT ISP people sometimes suggest) is to plug your internet connection into the phone master socket. This should establish whether or not there are any problems with your home phone wiring.