Thanks Merlin, just walked back upstairs with a big smile on my dial.
For future reference, the two brake bracket retaining bolts are 6mm with a 12mm head
The flathead screw I could see (and undid ) turned out to be a retaining pin for one of the handbrake shoes. And those undersize, grooved, pitted, corroding rotors are happily going into the bin!
Is it worth giving the handbrake shoes a light sand to tidy them up? THey are covered in all sorts of filth. Maybe just a squirt of brakekleen?