I've got to replace the outer sills on my mk2 coupe but need to know if there is an easy way of seperating the outer sill from the inner sill without causing damage to the inner sill. Are they spot welded together?

Seam welded IIRC. The best way is to choose your new seams location and get the grinder on the case.

I've recently done this.

Rub some sandpaper along the seam until the spot welds show up,and drill them almost deep enough to free the outer panel with a big drill bit,like 8-10mm.

Wedge a wide,cheap,chisel in there to split the sheets,after. A thin cutting wheel will cut the rest fine,but go as shallow as you can.

Tip! Before welding on a new panel,clean the rustproofing wax from the inside of the remaining panels,up to four inches from the weld edge. This is because heat from the weld will melt the wax and cause it to drip down into the weld pool and weld edges,causing a total nightmare. The wax also catches fire,and my inner rear quarter panel was alight a few times..

once its all welded up,you can re-wax it from behind the interior rear trim panel.