I hope you're still tuned in to this thread Mark. I've just dug out the dimensions I took for my RORC CH rating for the RIOW race in 1985!
The main ones you need for ordering a spinnaker are I which was 23' 4" and J 10' 0" These resulted in a spinnaker of SL (luff length) 26' 4" and SMW (max width) 17' 4" which seemed to work pretty well.
The other measurements which may be worth putting on record are
LOA 21' 9"
LWL 18' 0"
Beam 7' 9"
Draft max/min 4'/2'
Displacement 2600lb
P (main luff) 22'9"
E (main foot) 9' 3"
LL (forestay) 27'4"
SPL (pole length) 9' 1"
LP (genoa perpendicular) 13'
We got a Channel Handicap TCF of 0.730
Geoff ex S/287 Zalophus