Hmmmm, this isn't what I heard before, but never mind:
The third mishap to a Duchess occurred on 24 January 1962, and was
the last boiler accident on BR. 46238 was hauling a Holyhead to
Euston express when, approaching Bletchley, part of the roof of the
inner firebox tore away from the stays and released the contents of
the boiler. One water gauge glass was so dirty that it was impossible
to see the water level and the other one showed what the fireman
believed to be a full glass, the water level not being visible. In
fact it was an empty glass (this mistake could not have been made
with an LNER water gauge). The enginemen survived.