Well... it's done. I had to fiddle about with the tuning, and in the end it seems best in fourths, to a', d', g' and c''. There's a bit of buzz on the open strings - it doesn't spoil the sound, but I wonder if it's originating in the nut. Here it is with my clumsy fingers playing it (bit of a sloppy recording on the computer microphone). It will meet its new owner, my sister, next weekend.
The top pair of strings are still missing, but I have now had a go at tying the gut frets. This mostly went well, but I had a couple of breakages (actually the knots slipped and it's impossible to reuse the piece of gut) and I ran out of material! I had to use a length of nylon string as a temporary fix for the seventh fret.
The cittern was dusted off, and I made and fitted the braces to the back.