There was something on the teevision recently about The Mallard.
I believe it still holds the speed record for a steam train.
126m.p.h. some time in the thirties .......might be best if I google that to be sure of my facts.

Yes indeed; 126mph on 3rd July 1938 with 'A4' class LNER Pacific 4468 'Mallard' with a seven coach, 236.5 ton train (far from the heaviest!!) for five miles on Stoke Bank.

The engine driver Joseph Duddington (retired 1944) during high speed brake trials and no doubt as an attempt on the record as a publicity stunt, to acquire an advantage over their rivals the LMS, was done at significant risk, with 'Mallard' actually sustaining severe damage. 
It is argued that the A4 class was the only locomotives in the UK to be able to achieve 100mph in everyday running.
100mph club
Castle
King
Merchant Navy
Duchess
Princess
A1
A3
A4
There will be more I think that Britannias and Duke of Gloucester are members too
Best member of the 90mph club is the 9F
I knew my comment would cause a comment, but I did say "argued" Martin, as engineers, historians and enthusiasts alike have many different views over the ability of many fine engines, some of which you rightly list, over their ability to achieve and maintain certain levels of speed, let alone 100 mph, along with efficient output in "everyday use".
Many variable factors, such as ability to raise and maintain steam pressure - boiler design - firebox grate area - superheater efficiency - steam pipe design efficency - blast pipe and chimney layout, etc, etc, along with bearing surface areas, valve gear design, piston / cylinder layout / travel, then wheel size and arrangement, with the overall weight consideration are, as you no doubt know, all contributing to the complicated and often controversial "package" of whether or not engines can achieve 'continual' maximum output efficiency on the main line.
Then you add the variable of the individual engine drivers and fireman along with the geography of their particular line routes, and it is not surprising many 'argue' over which engine and class was actually the most reliable, efficent, and fastest out of them all. Personally I certainly believe, given the 'right' situation, the 'Kings' and 'Duchess's could always have equalled the 'A4's' in everyday running.

The 'Merchant Navy's' could only do so once rebuilt, but any form of "everyday" 100 mph performance was only achieved occasionally by drivers out to give them a final thrashing before withdrawal. The 9F's on "everyday running" I doubt due to their 'freight engine' layout, and especially on 10 coupled 5' 3" wheels. :-/ :-/ After all they were not designed for speed but long term power output.
This argument will no doubt go on indefinately (especially as new facts are established such as with the 'ashpan draughting flaw' of 8P Standard Class 71000 'Duke of Gloucester' discovered a few years back during restoration) but we as knowledgable historians / engineers / enthusiasts must accept the fact that the 'A4' class does still hold the World steam railway engine speed record of 126 mph which the LMS, GWR and Southern (or later BR!!) failed to equal or exceed with their engines, and the A4's did run as true 100 mph locomotives in "everyday use". 8-)