As excellent as it is the Mullard GZ34 is not recommended for long term use in the HMV 1804 or any of the variants of this model such as the pre-war model 1800 and Marconi VT50.
It is the much lower forward voltage drop in the GZ34 that causes problems, with this valve the HT voltage can be over 400 volts. The 5U4/U52 has a forward voltage drop of 44V @ 225mA. It is obvious the designer had factored this in during the development of the chassis. If one can be found the best valve for the job is the Mullard GZ33 or Cossor 53KU.
http://www.radiomuseum.org/tubes/tube_gz33.html
I'm now using a Canadian made Westinghouse 5U4G in the set.
The attachment shows a comparison between the GZ34 and 5U4G.
DFWB.