Hi RR, The Pye 405 line colour set we are discussing employs three DY86 diodes in what is not strictly a double circuit. The EHT over winding on the transformer provides 17KV which is augmented from the 6KV pulse present at the anodes of the two PL36 output valves to produce 23KV. It's the same arrangement is used in the RCA 21CT55. The 21CT55 was developed from the CT100, the first RCA colour TV.
EHT boosters were used on much earlier sets. For example the post war HMV model 1804. When it came to replacing the original Emiscope 3/4 CRT, the TA10 was the replacement. This tube required more EHT, the mains transformer supplied only 4KV. The answer was an EHT booster which used the 1-5KV pulse present at the anode of the line output valve to augment the existing 4KV. You got 5 to 5.5KV.
I'm certain that the DY86 valves are really not up to the job. Maybe something more like this: http://www.radiomuseum.org/tubes/tube_3a3.html
Used in American colour TVs.
The DY86: http://www.radiomuseum.org/tubes/tube_dy86.html
The DY802 could be considered.