The gassy KT44 has been replaced by an Osram KT45, which is essentially the same valve but has better peak anode voltage ratings. The KT44 was a development of the KT66 audio output valve. The KT44 was first used in 1938 for service as the line output valve in the HMV 902A TV and radiogram combination.
Info about the KT45:
http://www.r-type.org/exhib/aad0234.htm
I can confirm that the line timebase is now working and is supplying the scanning coils with a 100microsecond sawtooth waveform.
The next stage of the restoration of the 1804 will be attention to the video amplifier and sync separator stages.
The attachment shows the similarity between the KT66 and KT44 valves.
DFWB.