My Dual 867 is with a comapny called 'Amsys' near Heathrow, where it was sent by Apple...
Avoid them like the plague!! It pains me to even hear their name mentioned...
Regardsless of time, I think Cancom is a pretty good bet as they are well known and would have a lot of experience... I'm pretty sure they used to offer a service where you could pay an extra premium and bypass the cue...
There is also a dealer/Applecentre on Oxford Street that offers the same service...
Peace,
Marc
*Don't know if the part would be compatible with the QS machines, but it may be worth asking around to see if anyone has a spare supply after a Powersupply exchange swap out...*