When our kitchen was done recently the granite suppliers provided a
window sill that was the wrong size - their fault, but they left the
old one behind.

We have a window that it will fit, which currently has no sill. The
wall upon which it would sit is of lightweight concrete blocks with a
plasterboard face. Unfortunately, the horizontal surface on which the
sill would be mounted is not very well finished, and is not level. A
builder friend suggested dabbing compound and plasterboard to bring it
up to level, then laying the sill on that surface.

Is there any alternative to "dabbing compound", whatever that is? Do
I have to buy half a hundredweight just for this tiny job, or will
something else do instead? Preferably something I can buy at a Shed,
in a reasonably small quantity.

Alternatively, is there an alternative to the method proposed?

Thanks

Edward