5381A update.
Firstly a reminder of how it turned up, which is to be expected with vintage LEDs really. Technical term: useless.
Of course I found the thing with a bodge repair that wasn't done properly and used the wrong displays.
Some reverse engineering later and new standing current for LEDs was picked. Settled on 220 ohms in the end which has higher brightness with the new high efficiency red LEDs for much less power dissipation overall.
For the sake of people landing on this via Google one day, the new blue resistors are Royal Ohm 220R 1% 0.25W and Displays are Kingbright SA03-11EWA which are drop in replacements.
If you do futz with these, make sure you run with the lid on as there's live mains in easy accidental brushing distance of the top of the chassis.
Quickly popping it on myself as a handy 50Hz antenna
The display looks clearer in real life. The phone can't cope with high contrasts like that
. I really need to buy myself a DSLR again...