It would appear I've bricked a CECHC03 whilst attempting OtherOS++ install. A quick google reveals that this is happening to a few others.
Rebug 3.55.1 with OtherOS++ modded in.
Followed instructions on Gitbrew and everything appeared to go perfectly (bleeps etc) on first boot to petitboot PS3 appears to come up as normal, beeps in correct places etc. but there is no output to TV.
Tried hard reset, tried to boot to service mode... Same deal.
This would appear to indicate that I've fried the NAND and have a nice shiny black doorstop.
As I said I've noticed a few other examples of this happening so it's worth bearing in mind.