The rear mech is stuck and siezed into my (father inlaws!) frame. The mech is a old suntour SVX and the frame is steel ('58 Flying Scot). Anyone got any tricks up their sleeves?
Penetrating oil of some kind is always the first step, and plenty of time to let it soak in. A bit of heat can work as well but it usually depends on whether you want to save the paint or not.
You could also try tightening it prior to loosening, this has worked for me before. Failing that get some extra leverage on whatever tool you're using, probably best with the wheel clamped in to give the frame some support.