So far so good - all the rotten wood stripped out and new wood being cut
ready to fix in to the cut out sections.

What would be the best waterproof glue/sealer to put on the bottom of the
cill to stick it to the concrete base - I was going to use a combination
of long screws from the inside and ordinary wood glue to pull the new bits
in tight which is fine laterally but not horizontally.

but not vertically . . . .

DOH!