Thank you everyone for your views and opinions.
All this aluminium stuff is very interesting, but I don't have an aluminium flywheel and wasn't intending on buying one.
I already have the items stated and I wanted to know which one would be best. The steel item is quoted as being 40lb on the McLeod website. That seems very heavy for a lightweight Cobra. The cast one is part number C80E-A.
Thanks again.
Paul