The time had come (the Walrus said ...) to fit the chain plates. The ridiculously expensive 82 deg countersunk arrived and the mock-up for the offsets seemed to work. The definitive offsets were marked onto the hull planking.
First measure a couple more times and then centre punch the location for the 3 holes in the chainplate. Drill all three with a 2.5 mm drill and offer up the plate to the offset lines and hold very firmly in place.
Mark the position of the chainplate holes on the planking by drilling through the 2.5 mm holes just drilled. Remove the chainplate and replace with the 2.5 mm guide block pre-drilled on the pillar drill - this would ensure the holes would be perpendicular to the planking.
Drill through the planking to the extent of the drill's length and finish off through the frame with a spoke drill. A quick check on the inside confirms all holes are in line and through the centre of the frame. Bingo.
Next, enlarge the holes 0.5 mm at a time until 9.5 mm.
Back to the drill press and enlarge the holes in the chainplate to 9.5 mm and countersink to 9/10 of the finished depth.
Offer up the chainplate to the hull and check the bolts pass through the chainplate and hull whilst still lodging just under the plank lands - tight but good. Success. Only 3 more to go.