OK so I nipped home from work for an hour (great company!) to take a look at some Lv2diag.self files I found on the wiki.
Tried them all on the root of my USB HDD and none of them worked.
I have tried with the 3.55OOS++ pup in the root of the drive as well as the normal update folder. Getting nowhere.
This is the same HDD that I used to fix the RSOD issue and that worked fine.
Would it be worth trying to downgrade from 3.55 OOS++ to a lower firmware to get out of FSM?
I could then start afresh with 3.55CFW.