Andy,
The SailJuice Winter series are run off the 'Great Lakes' handicap numbers which give;
RS Aero 9 = 1031, RS Aero 7 = 1059, RS Aero 5 = 1101
as opposed to the 'RYA PY' numbers which give;
RS Aero 9 = 1030, RS Aero 7 = 1065, RS Aero 5 = 1105
An explanation of Great Lakes numbers is here;
Different method and different data, but it produces very similar numbers which is encouraging
The RYA PY numbers are produced just in time for the UK Dinghy Show each year, which is normally the first weekend of March.