I was surprised by the firmware version you quoted, but you are right - the current version on the SpeedTouch UK web site
http://www.speedtouch.co.uk/codepages/content3.asp?c=7&ProductID=511
is indeed 5.4.0.11 (ST585v5UK_540B.bin, dated 24.01.2006).
But I have been running 5.4.4.1 (ST585_5441_UK.bin, dated 13.04.2006) on my ST585v5 for several months now, and I'm sure I downloaded that from the SpeedTouch UK web site. It is very stable (I checked just now and I've been synchronised for 32 days so far at 5,376 - not bad for a line on which I was refused an upgrade to 2 Mbps because "the line cannot support it").
It looks to me like 5.4.4.1 has been pulled, but why...?
JPL