Thanks for these great answers, and thanks jc101 i now have the micsig diff probe and its very good.
Of course, when i probe the current sense resistor (burden) of my PFC it is just full of pickup as these diff probes have each probe on separate lead......so i think i will still need to buy an isolation transformer so that i can use a home-brew coaxial probe to look at it without so much common mode junk in the way.
Alternatively i woudl like to take the back off the scope, measure the DC output of the offline SMPS... disconnect it, then feed in a isolated DC supply and then i have a floating scope.
Its a Tenma 72-8225