George,
you can do that easily with gr-trellis.
just define an FSM describing the Manchester code and
use the VA to decode it. You can actually do
soft decision demodulation on this as well, ie,
from the observations r_{2k} r_{2k+1} directly to b_k.
Take a look at some of the examples in
gnuradio-examples/source/python/channel-coding
Achilleas