62kbps to 5.3mbps, with no regularity, just totally variable.
If you are on a RADSL (Rate-Adaptive ADSL ) service, read the smallprint, you'll find there are no guarantees of speed .... no matter what your ISPs marketing department said about 8Mb !
removed a 500k Fixed limit from my service
How far are you from your local exchange ? Did you have issues when the line was first activated ?
If your connection got referred to an internal BT department known as the Long Line Team, when they've exhausted all other possibilities, they'll ask BT Wholesale to cut your line down to 500k, as sometimes it's the only way to get things working when you're far from the exchange.