O.k. Found the cause.
I was using 1.04 CT60TOS and found that I'd left the switch to bus boost on as I have an original CT60 with the F030 boost. Then flicked it off and the machine wouldn't boot without it. Just kept resetting.
I read in the manual that 1.04 had some problems, so I tried 1.03c and hey presto. We're back in business
Unfortunately 1.03c disables my CTPCI
So tried 2.01, and it worked for the first two resets, and I'm back to a black screen at bootup. I don't think this is related to the SV though, I've always had this problem with the 2.0x series.
So currently, I'm stuck with 1.03c with the SV and no CTPCI
Falcon CT60, Falcon CT63 x2, TT x3, MegaST x2, MegaSTE x2, STFM x2, STE x2, STacy, STBook, (Dead) Hades 060, Milan 060, T40.