The last New Era meeting I visited had the Hornets, CB500s, 350 power valves, 125s, Formula 400 and one SV650 all running at the same time, but I don't think there was a actual class for the SV.
Here is a link to the North Glos Mini Twins regulations that the SV650s compete under,
http://www.ngroadracing.org/Pdf/minitwins.pdf the Desmo Due regs provide for a wider range of modifications. So as I see it North Glos only have to apply their origianl regs to the 4 valve per cylinder twins and apply our regs to 2 valve per cyl twins.
With 72 bhp the SVs may still have an advantage but as some of those bikes are costing £4K plus thats to be expected