The Elstone mains transformer is interesting though, and quite likely an old item (in 1956) reused.  It has a 5V winding, with tapping at 4V, for the rectifier heater.  Whilst 5V heater rectifiers were common in 1956 I reckon that 4V heaters were pretty much obsolete except in equipment designed a decade earlier, e.g. some test equipment.  I'd like to use the smaller EZ80 rectifier for this project, so a 6.3V heater winding for the rectifier is required.   

Links

 http://www.vintageradio.me.uk/amplifier/mullard33.htm

 http://technicalscientific.com/data/Mull_3W.pdf   from http://www.technicalscientific.com/techinfo.htm

 DiyAudio thread on merits of circuit http://www.diyaudio.com/forums/showthread/t-93872.html