Hopefully someone can tell me if this is feasible. We've recently cut
away flowerbeds from a (newly) fully turfed back garden, and currently
have all the excess turf piled up in one corner.

Our idea was to let this die off and hopefully let it break down so
that in the spring we can use it as topsoil for the flowerbeds.

The turf that's in the middle of the pile has started to die, and i've
re-piled it so that the stuff at the top that's still alive is now in
the middle etc, but is it feasible to expect this will have broken
down enough to use in the Spring? Should I be using any sort of grass
killer? Or is this a daft idea and i should just dispose of this and
order some topsoil?!

Any advice would be most appreciated, thanks