If you've confirmed -ve side of the HV cap is common with the secondary 0V then that should be fine, on P7 of the datasheet it seems that topography is used.
There's enough info in the datasheet to determine the likely fault and without the PSU in front of me the main suspects I have: Opto, diode, cap and resistors in relation to the Control pin or the TOP224P itself.
Not a lot to check with a DMM and if the passives are OK suspect the opto or TOP224P.
You should be able to confirm if the opto is OK with your DMM, that just leaves the TOP224P.
One more thing.....check the transformer primary is not OC.
Ok, passives look fine. I have measured the optos and get odd results as follows:
U1, PC817
pins 1-2 (+ve on 1, -ve on 2) = 1078
pins 1-2 (+ve on 2, -ve on 1) = 620
pins 3-4 (+ve on 4, -ve on 3) = OL
pins 3-4 (+ve on 3, -ve on 4) = OL
U3, PC817
pins 1-2 (+ve on 1, -ve on 2) = 1068
pins 1-2 (+ve on 2, -ve on 1) = OL
pins 3-4 (+ve on 4, -ve on 3) = OL
pins 3-4 (+ve on 3, -ve on 4) = 635
I have a feeling my results are because the DUT is in CCT, thoughts?