Damp towels or sheets on top of the metal work well for damping the noise, just remember to lay an old one on the metal before you put your best bathroom set on there!
And if you haven't already tried the jigsaw, don't - it's about a million times louder than an angry grinder!!
A jigsaw will only be loud if you are cutting something like a barrel and at too high a speed. Plate held down firmly and cut on slow speed will not be noisy at all. Cutting too quickly will destroy blades too. Grinding will burn and maybe workharden your work then damp rags will send it rusty. You can't cut anything but the roughest straightish line with a grinder and it will look a mess whereas a jigsaw will cut out intricate shapes with just a light burr to remove, or a straight line that is straight. Some work needs a bit of finesse and it wont set fire to your shed.
Am I to understand that cutting or drilling steel needs to be done at lower speeds than when doing wood or plastic? how low? Do you guys use WD40 or engine oil to lubricate the cut? Any other tips? My work would mainly be 1.2mm with only bits of 3 or 4mm.
I have been offered 2mX1mx1.2mm CR Steel for apr £36.00 is this the right price for the stuff??
I guess you could place two 6X4s on the lawn to cope with the saw stroke, drape it with the steel sheet, keep it pressed to the lawn and then jigsaw it. (note recurring lawn theme - worx for me).