As an experiment, can you disconnect one leg of the diode CR5 and the Collector of transistor Q2.
Turn the brightness control to minimum. The screen should be completely blacked out because the CRT's cathode will be sitting around +33v and its grid around -30v (making the relative voltage grid negative with respect to cathode about 63v). You could also check that with the meter.
Then turn the VDU off and see if the bright flash has gone away, or still appears at turn off ?
(I'm wondering if there might be some leakage in the diode CR5 or Q2, but if the effect is still there, it will require some investigations of the way in which the grid-cathode voltages are falling at turn off)
This is the same VDU unit which had problems with Q1 and some diodes in the area, and a failure of Q2 to switch correctly ? if this is the case there could be more problems in this area. It might be worth replacing Q2 and CR5, but first lets see what the experiment shows.