Thought I'd just put this one up on the forum so that other people can find it if they encounter a similar problem in the future.
Device: Lenovo/LiteON PS-5281-01VF (different to the -7VWN) Slim form factor 250~280W PSU
Fault:
No power up, nothing, tester doesn't power up either
+5VSB (Purple) only showing 0.9V
Work done:
1) Opened up unit, noticed a couple of the main DC output caps were leaky, replaced those; Still won't start
2) Diode check on 5VSB+ secondary side parts all comes back good
2) Used hot air station at 220'C and started warming up parts, located faulty area, turns out area on primary side of smaller transformer is sensitive, replaced 22uF 50V electrolyte (no visible defects) and finally the PSU is working again.
Sounds simple looking back, but that took me all too long to work out. Sometimes I wonder if it isn't easier to start with tests like this before anything else. Almost guaranteed though no matter which method you start with, it will be the wrong one
As a bonus, the fan was turning slowly, and it was a 90mm unit (not 80mm) so I threw that in to the ultrasonic cleaner and relubricated it, good as gold again.
Normally I'd not bother repairing PSUs, but this one I couldn't get a suitable replacement quick enough at a sane price in the country... so repairing it was.